
Please be aware HMRC have started migrating customers to a new numbering system for the Period Keys they use. Presently this can mean that you will see two of the same period in your Obligations dialog as Khaos Control will have received earlier versions of period keys for the same period which has now been given a new number:
One in the form NNNN and one in the form NNAA, e.g.
If you do encounter two obligations for the same period, you should always select the period which is in the form NNAA otherwise you will encounter a “Invalid Period Key” error returned from HMRC.