I think that the key is “inform about crossposting ...” since many of us frequent many sites.   Also, if you do find a particularly-useful reply on another site, I also like it when people mention (and summarize) the relevant posts here along with a cross-link to their sources.   (The summary being intended to maintain the information-value here no matter what “those other sites” might do, say, five or ten years from now ... if they still exist ;-) ... and if, ugh, I “still exist.”)