putting _design docs if they are already present result in bigcouch "nocatch,conflict" 409 error

while registering a new user on dev.bm we get this on urchin:

[Mon, 06 Jan 2014 13:31:18 GMT] [error] [emulator] [--------] Error in process <0.547.180> on node 'bigcouch@urchin.dev.bitmask.net' with exit value: { {nocatch,conflict},[{fabric,update_doc,3},{chttpd_db,'-update_doc/6-fun-0-',3}]}

(from redmine: created on 2014-01-06, closed on 2014-02-05, relates #6456 (closed))