Does the ー represent a double vowel or a long vowel sound?


For words like パーティー (party), I know ー is doubling the ア and イ. But for a word
like センセイ (and let's assume for the sake of argument that it is regularly
spelled in katakana), would it be センセイ or センセー ?




The fact is, 先生 is not regularly written in katakana in the first place, so
**there is no authentic guideline that tells you which is correct**. If you
are going to write it in katakana, there must be an unusual purpose for doing
it. If that purpose is to indicate that you just don't/can't use kanji, then
センセイ would be appropriate. If the purpose is that you wanted to make it look
like it is pronounced by a foreigner or a robot, then センセー might be more
appropriate.

