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.