Many languages lack phonemic glottal stops, but regularly insert them. For example:
English invariably inserts glottal stops before utterance-initial vowels, and often before word-initial vowels when enunciating:
/ˈɔw ˈnɔw/ [/ˈɔwˈnɔ́ẁ/] 'Oh no!'
and also when needed to break up adjacent identical vowels:
/ði ˈir/ [ði̠ˈʔiɰ˞] 'the ear'
Japanese is similar, except it also allows a glottal stop utterance-finally, especially in emphatic utterances:
/ee/ [ʔèéʔ] ⟨ええ?⟩ 'huh!?'
This seems to be an under-studied phenomenon despite its widespread occurrence. Do you have any references comparing how this works in different languages?