A little background:
When you create a payment in Cliniko but don't allocate it to an Invoice, it creates account credit. When that account credit is allocated to new Invoices, the original payment still shows as "unallocated" because it was used to create the account credit. Before the bug fix, you were actually able to edit these payments and reallocate them from account credit to a new invoice, even if you already used the account credit that payment had created.
This has now been fixed, and you can no longer over allocate payments.