Sounds like OP wants to replicate some data for each of existing users but add only once per user. Seems like currently table is having multiple records per user.
If thats the case, it should be something like
INSERT INTO YourTable (UserID,.. other columns)
SELECT UserID,
value1,value2,..
FROm (SELECT DISTINCT USerID FROM YourTable)t
if this was not your question then please post some sample data as James suggested as we cant make out much from your current explanation.
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SQL Server MVP
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