limit Db insert on ID

This worked for another table but does not seem to work for this insert statement, basically i don't want to insert into the other table if the SSAN already exists in the table.  What amd I doing wrong.

Insert Personnel.dbo.tblPersonnel (strSSN, strLName, strFName, strMName)
Select	LTRIM(RTRIM(Replace(SSAN, CHAR(160), ''))) SSAN,
		Substring(Name, 1, charindex(' ', name, 1)-1) Last,
		Substring(Name, charindex(' ', name, 1)+1, charindex(' ', name, charindex(' ', Name, 1)+1)-charindex(' ', name, 1)-1) First ,
		Case charindex(' ', Name, charindex(' ', Name, charindex(' ', Name, 1)+1)+1) when 0 then '' else 
		Substring(Name, charindex(' ', Name, charindex(' ', Name, 1)+1)+1, charindex(' ', Name, charindex(' ', Name, charindex(' ', Name, 1)+1)+1)-charindex(' ', name, charindex(' ', Name, 1)+1)-1) End Middle
FROM	[Personnel].[dbo].[tblTemp] where not Exists (Select null from tblPersonnel where SSAN = strSSN)

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