In the Lookup stage the field names of the lookup keys should be the same and the other fields (in Reference and main link) should be unique. Like JoshGeorge said try disabling RCP or rename one of the fields.
It is our choices that show what we truly are, far more than our abilities - J.K.Rowling
You will get this error when you have the same column names in the input, reference and output links. As maveric said try to rename the outputs coming from reference link differently or if you don't need any data coming from the reference links try to delete the columns that are not key columns.
Example1:
Input:
Col1 (Key)
Col2(Key)
Col3
Col4
Reference:
Col1 (Key)
Col2(Key)
Col3
Col4
Output:
Out1 (map) -------- Col1 ( from Input)
Out2 (map) -------- Col2 ( from Input)
Out3 (map) -------- Col3 ( from Input)
Out4 (map) -------- Col4( from Input)
Out5 (map) -------- Col3 ( from Reference)
Out6 (map) -------- Col4( from Reference)
Example2:
Input:
Col1 (Key)
Col2(Key)
Col3
Col4
Reference:
Col1 (Key)
Col2(Key)
Output:
Out1 (map) -------- Col1 ( from Input)
Out2 (map) -------- Col2 ( from Input)
Out3 (map) -------- Col3 ( from Input)
Out4 (map) -------- Col4( from Input)