An ELT Glossary : Gemination

Gemination is a feature of connected speech. When two identical consonant sounds occur adjacent to each other one syllable final and the other syllable initial - they will often merge into one. This geminated consonant is slightly longer than normal, but it is impossible to say to which syllable it belongs - it seems to belong to both at the same time. Examples :

I want twelve of  those zinnias, and seven nasturtiums. I've got a big garden.

/aɪ wɒntwelv əv ðəʊzɪnɪjəz ən sevənæstəʃəmz aɪv gɒt ə bɪgɑːdən /

See also : Sandhi variations