cubicweb #1695 keep bad session data when trying to add an already existing relation [open]

I've tried to add via the web form a relation which was actually already there, so I got a "duplicate key violates blablabla" error. Fine.

But after that I get the same error when I try to add another, unrelated, entity (of the same type).

To fix this I had to reopen a fresh session.

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load left0.500
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