What is the meaning of uppajjati versus upapajjati?

In the Pali there are two terms, which are (generally) used contextually in two different ways: 1. uppajjati (and its past particle uppanna) 2. upapajjati (and its past particle upapanna) What do these terms actually mean and what is the distinction between them? Importantly, how are these terms constructed or derived in Pali to differentiate their meaning? Of note: the PTS dictionary makes the rather vague comment: > *doubtful whether a legitimate form as upa + pad or a diaeretic form of uppajjati = ud + pad. In this case all passages ought to go under > the latter. Trenckner however (Notes 77) defends upa˚ & considers in > many cases upp˚ a substitution for upa. The diaeresis may be due to > metre, as nearly all forms are found in poetry. The variant reading > upp˚ is apparently frequent; but it is almost impossible to > distinguish between upap˚ and upp˚ in the Sinhalese writing, and > either the scribe or the reader may mistake one for the other*