Fix #1794
Fix #1794

This doesn't tackle the root cause (which is that the mixin gets added
twice despite only being declared once) but it does at least ensure that
the duplicate mixin doesn't get an extra copy of the the attributes.

cesarmarinhorj/jade,VANITAX/jade,harp/jade,harp/jade,mumaoxi/jade,magicdawn/jade,ridixcr/jade,tranhieutt/jade,Mitrend/jade,ceremcem/jade,Shinchy/jade,mrotaru/jade,rajnishyadav321/jade,webskyer/jade,martindale/jade,modulexcite/jade,andreytkachenko/jade,VeraWei/jade,angryrobot/jade,pugjs/jade,angrilove/jade,ilovezy/jade,Shinchy/jade,fredyteheranto/jade,Victorgichohi/jade,jodytate/jade,jadejs/then-jade,pugjs/jade,Laoujin/jade,mrotaru/jade,gexiaowei/jade,cgvarela/jade,webskyer/jade,jirengu/jade,colin-h/jade,jjz/jade,jadejs/then-jade,ridixcr/jade,cesarmarinhorj/jade,pugjs/pug,illusionfield/jade,jirengu/jade,muthhus/jade,ilovezy/jade,webmasteraxe/jade,VANITAX/jade,coderhaoxin/jade,bogus34/jade-vfs,angrilove/jade,cutsin/jade,mumaoxi/jade,Laoujin/jade,webmasteraxe/jade,muthhus/jade,jodytate/jade,VeraWei/jade,magicdawn/jade,angryrobot/jade,cutsin/jade,colin-h/jade,bogus34/jade-vfs,gdibble/jade,pugjs/pug,illusionfield/jade,coderhaoxin/jade,jjz/jade,rajnishyadav321/jade,Victorgichohi/jade,pugjs/then-jade,pugjs/then-jade,fredyteheranto/jade,omixen/jade,omixen/jade,gexiaowei/jade,hnag409/jade,tranhieutt/jade,andreytkachenko/jade,Mitrend/jade,modulexcite/jade,martindale/jade,hnag409/jade,gdibble/jade,cgvarela/jade