fix: throw when csv produces non-unique cmdt record names

[mshanemc]

forcedotcom/cli#2573
@W-14512810@

bug was coming from file cross-saves

QA:

repro issue with the CSV provided on 2573. You'll need to create the CMDT object/fields. Use this to get the records set up to match the csv.

sf cmdt generate object --label Table --type-name Table__mdt 
sf cmdt generate field --output-directory force-app/main/default/objects/Table__mdt --type text --name TelelinkInvoicingCompany__c
sf cmdt generate field --output-directory force-app/main/default/objects/Table__mdt --type Text --name TelelinkInvoicingCompany__c
sf cmdt generate field --output-directory force-app/main/default/objects/Table__mdt --type Text --name TelelinkCountryCodePick__c
sf cmdt generate field --output-directory force-app/main/default/objects/Table__mdt --type Text --name TelelinkCountryGroupCode__c
sf cmdt generate field --output-directory force-app/main/default/objects/Table__mdt --type Text --name TelelinkPostalCodeStart__c
sf cmdt generate field --output-directory force-app/main/default/objects/Table__mdt --type Text --name TelelinkPostalCodeEnd__c
sf cmdt generate field --output-directory force-app/main/default/objects/Table__mdt --type Text --name TelelinkVatSubjectCode__c
sf cmdt generate field --output-directory force-app/main/default/objects/Table__mdt --type Text --name TeleLinkCode__c