cubicweb-dataio #3479798 Multiple values handling in XY should be clearer [validation pending]

prioritynormal
typebug
done in0.4.0
load0.100
load left0.000
closed by#229398fab192 [xy] Clearer multiple values handling in merge_entity + proper warnings, closes #3479798
patchcleanup: clearer multiple values handling in merge_entity + proper warnings [applied]