FORALL alias variable was previously declared: <name>

You may not have a Dim statement for the reference variable of a Forall statement. For example:

You cannot declare a ForAll reference variable in a Dim statement. For example:

Dim x%
Dim fo(10) As Integer
Forall x In fo  ' error!

Either delete the Dim statement for x, or use a different name in the Forall.