How to re-declare T-SQL variables if it already exist?
You cannot redeclare variables, nor can you test to see if they have been declared (at least not that I have ever been able to find). BUT, you shouldn't need to do this anyway. If you know the variable names ahead of time, just declare them all at the beginning of the process. Then, use them throughout the script.

For example:

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-- BEGIN script header
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DECLARE @SomeVariable1 INT,
        @SomeVariable2 VARCHAR(50),
        ....;
        SomeVariableN DATETIME;
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-- END script header
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And then add 1 or more script blocks that use these variables:

---- Script Block 1 ---
EXEC dbo.MyProc @InputParam = 1, @OutputParam = @SomeVariable1 OUTPUT;
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---- Script Block 2 ---
EXEC dbo.AnotherProc @InputParam = @SomeVariable1;
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---- Script Block 3 ---
SET @SomeVariable2 = NULL; -- reset value if you don't want to carry over prior value
..do something here...
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