I can speak only for British English
I would say that it is partly regional (southern rather than northern), partly generational (i think it's far more common among younger people) and partly ethnic (more common in black communities).But lately—I’d say in the past few years—I have noticed a trend towards pronouncing such words in a different way, one that basically drops the “t” sound entirely and really emphasizes that second syllable, almost disconnecting it entirely from the first syllable, as though it were a separate word. It might be rendered in writing as “Clin’ ihn”, and it sounds like this:
It is a regional thing, or is it a generational thing?
I must stress that these are my impressions, not the result of any scientific research
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