In GA, a sequence of two identical consonants as in club badge, midday, pine nut tends to be about 1.5 times as long as a single consonant. Such ‘double’ consonants are therefore essentially pronounced as single consonants with a longer articulatory contact. This is called gemination.
Sequences consisting of identical fortis stops like those in pep pill, nighttime, bookcase will be realized as single glottalized, aspirated voiceless stops with a lengthened compression stage. That is, [ʔ͜kːʰ] will occur in bookcase, etc.
How can a single [kː] trigger off both glottalization and aspiration?
In Dutch double consonants are reduced to single consonants through simplification. There is no difference between the double /tt/ in (hij) gaat toch and the single /t/ in (ik) ga toch. Words like looppas, achttal, doellijn are pronounced /ˈloˑpɑs/, etc. Simplification also affects sequences of identical consonants which result from the application of various assimilations. In words like kopbal, bloeddruk /bb/- and /dd/-sequences arise as a result of regressive voicing. Subsequent application of simplification yields /ˈkɔbɑl, ˈbludrʏk/. Progressive devoicing creates a /ss/-sequence in leeszaal, which through simplification becomes /ˈleˑsaˑl/. Similar cases are roofvogel, jaszak, zeepbel. Be careful not to apply these AN rules to GA words. Pronunciations like */ˈʃɔbɛl, ˈbɛkraʊnt, ˈfaɪˈfoʊts/ for shopbell, background, five votes are strikingly foreign. A lack of familiarity with gemination may also cause perceptual problems as when phrases like I‘d do it, we’ll leave and they’ve found it are interpreted as I do it, we leave, they found it by Dutch speakers of English.