Row value updated or deleted either do not make the row unique or they alter multiple rows

Question

Hi friends
Iam using SQL 2005
By mistake I have inserted 5 rows with same Number.I have not  use Primary key
I have 2 delete other 4 rows So iam getting following error in SQL 2005
Error source:Microsof.visualstudio.datatools
Message:Row value updated or deleted either do not make the row unique or they alter multiple rows
What will be reason?
& What i can do 2 solve this problem?