I would recommend adding your key field to your data source prior to Tableau.
If you can idientify a distinct record in your data source by combining fields, you may be able to get the effect you are looking for by concatenating the fields together as a string with the "+" operator.
Another edge case is to turn off aggregation, un-checking menu option Analysis-> Aggregate measures, and use a INDEX() function to add a distinct number to each mark.
Thank you Joe!
I had a similar issue and everything worked great just by adding rownum on my custom SQL to pull in the data.
data source being an Oracle table.