MSSQL - Fix Error - Column names in each table must be unique. Column name 'IsValid' in table 'Test1' is specified more than once.

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To fix it replace

 Alter Table Test1
Add IsValid bit not null Default 0

with  

IF COL_LENGTH('dbo.Test1', 'IsValid') Is Null
Begin
Alter Table Test1
Add IsValid bit not null Default 0
End