Double Consonant.=Gemination.(wikipedia)

In phonetics, gemination happens when a spoken consonant is pronounced for an audibly longer period of time than a short consonant.
Consonant length is distinctive in some languages, for instance Arabic, Estonian(愛沙尼亞語), Finnish, Russian, Hebrew, Hungarian, Italian, Japanese, Latin, and Luganda. Most languages (including English) do not have distinctive long consonants. Vowel length is distinctive in more languages than consonant length.
Gemination in phonology
Lengthened fricatives, nasals, laterals, approximants, and trills are simply prolonged. In lengthened stops, the "hold" is prolonged. Long consonants are usually around one and a half or two times as long as short consonants, depending on the language. Consonant length is phonemic in Finnish: For example, takka [ˈtakːa] (transcribed with the length sign IPA: [ː] or with a doubled sign [ˈtakka]), 'fireplace', but taka [ˈtaka], 'back'.
In some languages, e.g., Italian, Swedish, Faroese(法羅語), Icelandic and Luganda, consonant length and vowel length depend on each other. That is, a short vowel within a stressed syllable almost always precedes a long consonant or a consonant cluster, whereas a long vowel must be followed by a short consonant.
In other languages, such as Finnish or Japanese, consonant length and vowel length are independent of each other. In Finnish, both are phonemic, such that taka /taka/ "back", takka /takːa/ "fireplace", taakka /taːkːa/ "burden", and so forth are different, unrelated words; this distinction is traceable all the way back to Proto-Uralic. Finnish consonant length is also affected by consonant gradation. Another important phenomenon is that sandhi produces long consonants to word boundaries from an archiphonemic glottal stop, for example otaʔ se → /otasːe/ "take it!"
Distinctive consonant length is usually restricted to certain consonants. There are very few languages that have initial consonant length; among them are Pattani Malay, Chuukese, a few Romance languages such as Sicilian and Neapolitan, and many of the High Alemannic German dialects (such as Thurgovian). Some African languages, such as Setswana and Luganda, also have initial consonant length—in fact initial consonant length is very common in Luganda and is used to indicate certain grammatical features. In spoken Finnish and in spoken Italian, long consonants are produced between words by sandhi effects.
Among stops and fricatives, in most languages only voiceless consonants occur geminated.
In various languages
English
In English phonology, consonant length is not distinctive within root words. For instance, 'baggage' is pronounced /ˈbæɡɪdʒ/, not /bæɡːɪdʒ/. Phonetic gemination occurs marginally.
However, gemination does occur across words when the last consonant in a given word and the first consonant in the following word are the same fricative, nasal, or plosive. For instance:
calm man [kɑːˈmːæn]
this saddle [ðɪˈsːædəl]
black coat [blæˈkːoʊt]
back kick [ˈbækːɪk]
With affricates, however, this does not occur. For instance:
orange juice [ˈɒrɪndʒ dʒuːs]
In some dialects gemination is also found when the suffix -ly follows a root ending in -l or -ll, as in:
solely [soʊlːi]
In most instances, the absence of this doubling does not affect the meaning, though it may confuse the listener momentarily. Notable examples where the doubling does affect the meaning are the pairs "unaimed" [ʌnˈeɪmd] versus "unnamed" [ʌˈnːeɪmd], and "holy" [hoʊli] versus "wholly" [ˈhoʊlːi]. (The latter two are identical in many areas, however.)
Estonian
Estonian has three phonemic lengths; however, the third length is a suprasegmental feature, which is as much tonal patterning as a length distinction. It is traceable to allophony caused by now-deleted suffixes, for example half-long linna < *linnan "of the city" vs. overlong linna < *linnahan "to the city".
Greek
In Ancient Greek, consonant length was distinctive, e.g., μέλω [mélɔː] "I am of interest" vs. μέλλω [mélːɔː] "I am going to".
The distinction has been lost in Standard Modern Greek, except in dialects such as the Cypriot-Greek dialect spoken in Cyprus, in varieties of the Aegean sea and elsewhere.
Hungarian
In Hungarian, consonant length is distinctive. For example megy means go, while meggy means sour cherry.
Italian
In Standard Italian, consonant and vowel length are distinctive. For example, "bevve" means "he/she drank", while "beve" means "he/she drinks/is drinking". Tonic syllables are bimoraic and are therefore composed of either a long vowel in an open syllable (beve) or a short vowel in a closed syllable (bevve). Double consonants occur not only within words but at word boundaries, where they are pronounced but not necessarily written: "chi + sa" = "chissà'" (who knows) [kis'sa] and "vado a casa" (I am going home) pronounced [va:do akkaza]. See syntactic doubling.
Japanese
In Japanese, consonant length is distinctive. For example, 来た(kita) means 'came; arrived', while 切った(kitta) means 'cut; sliced'.
Latin
In Latin, consonant length was distinctive, e.g., anus "ring" vs. annus "year".
Gemination still occurs in Italian and Catalan, but has been completely lost in French and Spanish.
Polish
In Polish, consonant length is distinctive. For example,
rodziny – 'of the family'; rodzinny' – adjective of 'family'
Grecy – 'Greeks' (noun); greccy – 'Greek' (adjective) — in fact it is pronounced [grɛt​͡st​͡sɨ].
Russian
In Russian, consonant length (indicated with two letters, as in ванна [ˈvannə] 'bathtub') may occur in several situations.
Word formation or conjugation: длина ([ˈdlʲinə] 'length') → длинный ([ˈdlʲinnɨj] 'long')
Phonological alternations:
высший ([ˈvɨʂːɨj] 'highest').[1]
Wagiman
In Wagiman, an indigenous Australian language, consonant length in stops is the primary phonetic feature that differentiates fortis and lenis stops. Wagiman does not have phonetic voice. Word-initial and word-final stops never contrast for length.
Writing
In written language, consonant length is often indicated by writing a consonant twice ("ss", "kk", "pp", and so forth), but can also be indicated with a special symbol, such as the shadda in Arabic, or sokuon in Japanese. Estonian uses 'b', 'd', 'g' for short consonants, and 'p', 't', 'k' and 'pp', 'tt', 'kk' are used for long consonants.
In the International Phonetic Alphabet, long consonants are normally written using the triangular colon ː, e.g., [penːe], though doubled letters are also used (especially for underlying phonemic forms).
In Hungarian, when two characters are put together to make a different sound, they are considered only one letter - for example, sz is one consonant that makes the sound [s] - a digraph. This is 'doubled' by writing ssz (rather than szsz), pronounced [sː]. The other digraphs cs, dz, gy, ly, ny, ty and zs work the same way: ccs, ddz, ggy, lly, nny, tty and zzs, respectively. The only Hungarian trigraph, dzs, can be geminated by ddzs. (B, c, d, etc. - 'bb', 'cc', 'dd', and so on.) The only digraph in Luganda, ny /ɲ/ is doubled in the same way: nny /ɲː/.
In Italian, the sound [kw] (represented by the letter Q) is always doubled by writing cq, except only in the word soqquadro where the letter Q is doubled.
Doubled orthographic consonants do not always indicate a long phonetic consonant. In English, for example, the [n] sound of "running" is not lengthened. Consonant digraphs are used in English to indicate the preceding vowel is a 'lax' vowel, while a single letter often allows a 'tense' vowel to occur. For example, "tapping" /tæpɪŋ/ (from "tap") has a "short A" /æ/, which is distinct from the diphthong "long A" /eɪ/ in "taping" /teɪpɪŋ/ (from "tape"). In Modern Greek, doubled orthographic consonants have no phonetic significance at all.
Catalan uses the raised dot to distinguish a geminated l from a palatal ll. Thus, paral·lel ("parallel") and Llull .