Duplicate type signature for x

This error message is caused by multiple type signatures for the same variable. It does not matter if the types are the same or different; there can only be one type signature per variable.

Disco> :{
Disco| x : N
Disco| x : N
Disco| :}
Error: duplicate type signature for x.

If this is unexpected, check that you did not misspell a variable name so it accidentally has the same name as another variable.