And the previous "purchase" of that product was on a different "invoice"? Do you still have a LineItems table?
You'll have to use your table name for the join (lineitems) table in place of mine...
If you add a self join relationship on WorkOut_Excercise, you can match by ExcersizeID and a ClientID field to match to all previous "purchases" of that specific exercisze by that specific client. Then it's simply a matter of linking to the "most recent" instance of that record.
Add a field to WorkOut_Excersize that auto-enters the ClientID from WorkOuts or Clients. (Do not use a calculation field as it will be unstored and that doesn't work for this purpose.)
In Manage | Database | relationships, make a new table occurrence of WorkOut_Excersize by clicking it and then clicking the duplicate button (2 green plus signs). You can double click the new occurrence box to get a dialog to appear where you can rename the new occurrence box as WorkOut_ExcersizeSameClientExcersize.
We have not duplicated a table. Instead, this is a new reference to the same table already present in your database.
Add it to your relationships like this:
WorkOut_Excersize::__pk_WorkOut_ExcersizeID ≠ WorkOut_ExcersizeSameClientExcersize::__pk_WorkOut_ExcersizeID AND
WorkOut_Excersize::_fk_ExerciseID = WorkOut_ExcersizeSameClientExcersize::_fk_ExerciseID AND
WorkOut_Excersize::_fk_ClientID = WorkOut_ExcersizeSameClientExcersize::_fk_ClientID
Specify a sort order for this relationship that sorts the records by __pk_WorkOut_ExcersizeID in descending order.
Then you can use a looked up value field option to copy from a field WorkOut_ExcersizeSameClientExcersize to get the data from the last time the client performed that specific excersize.