Consonants written twice always represent long consonants, not actual
repetation. eSpeak NG’s default behavior when speaking a doubled
consonant phoneme seems to work well enough for non-plosive consonants,
but for plosives, we need to tell it that the two input characters
correspond to one long phoneme, not a repeated regular one.

All three doubled voiceless plosives – ⟨tt⟩, ⟨pp⟩, ⟨cc⟩ – are regularly
found in Quenya, according to the Ambar Eldaron Quenya Dictionary [1].
Their voiced counterparts – ⟨dd⟩, ⟨bb⟩, ⟨gg⟩ – apparently don’t occur,
nor are any doubled plosives to be found in the Omikhleia Sindarin
Dictionary [2], voiced or not. But let’s define all six pairs in both
languages anyways, since it doesn’t cost us much to do so, and it seems
fairly clear that this is how these double consonants should be
pronounced, if they ever occurred.

[1]: https://ambar-eldaron.com/telechargements/quenya-engl-A4.pdf
[2]: https://www.jrrvf.com/hisweloke/sindar/index.html