Two RFx Response for same RFx created for single Vendor

Hi All

We are facing an issue where for a single RFx ,, there are two RFx responses are getting created by different two "Contact Persons" from SAME VENDOR.

Scenario :

Step1: User X(Project User) publish an RFx to the vendor V.

Step2 : User Y(Contact Person 1 of vendor V ) will open the RFx published and enters the data.

Step3: Same time when the User Y ,is not completed the RFx Response , the user Z(Contact Person 2 of vendor V) , also open's the RFx and tries to submit with details.

Step4: Now both vendor contact persons(belonging to Same vendor) will able to post two Rfx Response.

Note: The application will give the error if the User Y(Contact person1) will submit the RFx Response and User Z(Contact Person) .

So for any RFx there should be only on RFx Response from a single Vendor.

So kinldy let me know if any one has the idea's on this.

Thanks

CB