You can't have the same Subform in a form more then once. If it is not a computed subform it should warn you and not allow you to paste (IIRC). However if you have a Subform within a subform it might not pick up on this. 

Eg.  

1. SubForm1 
2. SubForm2 = Contains SubForm1
3. Form = Put on Subform1 and SubForm2 and save. Error will now happen. 

Are you able to open the form with the "Edit as DXL" option? Or modify the subforms to a blank one (rename and create a blank one). 

I copied it from another form and i was sure that there was not the same subform already in the form, however maybe i pressed ctrl-v twice.. but when i saved the form there was no error message.. The Subform itself is very simple (2 tables, 4 fields, no actions or other subforms)..

I tried editing with DXL, but as soon i edit the DXL, it seems that all the stuff after the subform is being removed.. after saving and reopening the form the form is blank..

I tried also access the form with an agent and remove the Subform Items, but no success..

Well i've given up, started again with the form.. luckily i could access the form title, form events and actions from DXL..

I tried to reproduce here but when I paste a second time the 9.0.1 Designer client just appends a _X to each field to prevent field duplication. 

Can you paste the DXL source of the sub form here? (make sure no confidential data), otherwise if you can open a PMR we can investigate further. 

Hi Simon

Unfortunately, i can't remember which subform it was.. I had also other selfproduced issues in the database when i changed some arrangements of the subforms / fields (double names fields in form / subform, design element inheritance).  So i think it's a mix of many things which resulted in this error..

kind regards,