Where in the old testament does it say that the messiah will return?

The OT is the Christian and somewhat Christianised version of the Jewish Tanakh.

Nowhere in the Tanakh does it say that the Jewish maschiach will come ‘twice’. How could he? He’s a normal mortal. Normal mortals live normal, mortal lives.

The idea of the ‘second coming’ is purely Christian and one might be tempted to think it was developed to explain how it was that Jesus failed to fulfill the 23 Jewish messianic prophecies before the Romans killed him.