Is there any difference between per mille (Q181011) and parts per thousand (Q99487704) other than the Wolfram Language unit code (P7007) value? This seems like a bug.
Topic on User talk:Toni 001/Structured Discussions Archive 1
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I've been wondering about the difference, too. So far I have not found a source which explains well the difference. I have the feeling that they, while having the same numeric value, are used under the name "per mille" or "parts per thousand" depending one the discipline. The former has symbol ‰, while the latter uses ‰ or is spelled out, but some disciplines do use "ppt" despite its ambiguity (thousand vs. trillion).
It's not uncommon for units to appear under different "names". For instance, coulomb and ampere second are both units of electric charge with a conversion factor of 1. They are equal but not the same. In Wikidata we have been preserving such "duplicates" which I think is good.
If we're not sure what the difference between them is, I'd say they should be linked with said to be the same as (P460), not different from (P1889).
I'd say that they could also have a said to be the same as (P460) statement.