Assuming this is the root area of the issue, and that for whatever reason the Set<DescribeResult> is no longer ensuring uniqueness, this puts the field name strings into a Set<String> that we're confident **is** unique and then builds the additional query string from that set.
Assuming this is the root area of the issue, and that for whatever reason the Set<DescribeResult> is no longer ensuring uniqueness, this puts the field name strings into a Set<String> that we're confident **is** unique and then builds the additional query string from that set.

SalesforceFoundation/Cumulus,SalesforceFoundation/Cumulus,SalesforceFoundation/Cumulus,SalesforceFoundation/Cumulus