You are replicating Materials from an External System to C4C and system throws below error message,
GTIN XYZ already exists in material ABC
GTIN XYZ already used for material ABC
The GTIN number is unique to a Material and it should not be duplicated.
A same GTIN number should not be used for more than one Material, hence remove it, if there is any duplicate entries.
Note: Also make sure whether you are entering the valid GTIN numbers for the Materials because system will check whether GTIN is valid as per the standards.
The system is working as expected.
KBA , SRD-CC , Cross Components , How To