The assignment to quantity field & is unclear!

What causes this issue?

You have assigned more than one SD quantity field to a CO-PA quantity
field. Consequently the assignment is ambiguous. You can only assign one
SD quantity field to each CO-PA quantity field (1:1 ratio).

System Response

If you have more than one entry in the table of assignments, the system
only accepts the last entry.

Error message extract from SAP system. Copyright SAP SE.