Unexpected 倍

Zさん："The kanji 旧 is interesting. It means ex-something, and is pronounced きゅう。I would expect it to be pronounced じゅうはち。" 

ハハ

Zさん：”Happy Mother’s Day!”

Bさん：「ハッ？」

Zさん：”今日はハハ”

Bさん：「Allow me to reiterate, ハッ？」