Custom lookup select issue

The idea is to contruct a lookup with unique Employer name. The lookup works fine but when I select a value and then select the lookup button again and click on the place marked in RED, there are duplicate values, which is wrong.

Kindly refer the snippet and snapshot

QueryBuildDataSource    qbds;
Query                   query = new Query();
FormStringControl       control = dialog.formRun().controlCallingMethod();
SysTableLookup          sysTableLookup =  SysTableLookup::newParameters(tablenum(VendTable), control);
;

qbds  = query.addDataSource(tablenum(VendTable));
qbds.addGroupByField(fieldnum(VendTable,EmployerName));
sysTableLookup.addLookupfield(fieldnum(VendTable, EmployerName));

sysTableLookup.parmQuery(query);
sysTableLookup.parmUseLookupValue(false);
sysTableLookup.performFormLookup();

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Solution

Make a view on table VendTable and field EmployerName and a count of RecId, then base the lookup on the view.

As shown below for CustTable and CustGroup:

enter image description here