Collaps values in colums if present in another

Question

Hello everyone one I have a dataframe such as :

COL1 COL2 COL3
G1   A
G1   A
G1   B
G1   A
G2   B
G3   C
G3   C
G4   OK
G4   A
G5   D
G5   D
G5   D
G6   F
G7   F