Hi Rajeev,
It looks a bit incorrect as Key field is changing in ECC and you are expecting MDM to not create a new entry but you are matching on the key field;this is bound to fail.e.g.How can you not create a new entry in Material table when Material no has changed and you are matching material number.
It would be better to define the scenarios possible and then change the map accordingly.
One option can be to do a complete replace of the data coming from ECC into MDM.
Thanks,
Ravi