Just to elaborate on eslang's excellent answer..
The doubling of consonants in the romanization coincides with the small ‘っ’ in the hiragana version. They both (small 'っ" and doubled consonants) mean the same thing - that there is a slight pause between "い" and "ぱい".
A small tip on how to approximate the pronunciation of the small 'っ' that helped me. Pronounce it as if the word is actually two words that are split in the middle of the doubled consonant (eg. ip pai). Even though I am putting the pause in what seems like the wrong place, the first consonant doesn't actually end up making a sound until you pronounce the 2nd part.
The doubling of consonants in the romanization coincides with the small ‘っ’ in the hiragana version. They both (small 'っ" and doubled consonants) mean the same thing - that there is a slight pause between "い" and "ぱい".
A small tip on how to approximate the pronunciation of the small 'っ' that helped me. Pronounce it as if the word is actually two words that are split in the middle of the doubled consonant (eg. ip pai). Even though I am putting the pause in what seems like the wrong place, the first consonant doesn't actually end up making a sound until you pronounce the 2nd part.