word riddles

[samirah]

there is a word which is singular when you add 's' add the end it become plurial. when you add another 's' it become singular again and when you add 'es' it become pluriel. what is it?

[sir.prize]

Prince

[Postdog]

Gubber, one I knew too!!.

BTW the word is plural.

[factor-fiction]

bra
bras
brass
brasses