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 sevənəstəʃəmz aɪv gɒt ə bɪgɑːdən /

See also : Sandhi variations