Form persistence issue when merging to the same collection twice

[twagoo]

To reproduce:

1. Make a new collection with a minimal number of references
2. Submit a number of resources, e.g. from the VLO. Merge these into the collection made in step 1. Submit the edit form.
3. Submit a new selection of resources, e.g. from the VLO. Merge these into the same collection as in step 1. The edit form does not contain the newly submitted resources!

[WillemElbers]

After testing and discussing with Twan, this seems related to a cached submission which is not cleared after submitting from a third party integration multiple times. The cache is only clear after cancelling the edit or using the "cancel submission" button in the merge submission page.

A successfull save does not clear the submission.