Well, just referring to this question,
Since the possible duplicate of it was on another site, would it also be closed as duplicate ?
Cross-site duplicates can be flagged for ♦ moderator attention.
Note, however, that sometimes its OK to have a apparently-duplicate question, because different sites focus on different things. E.g., "how do test the latest kernel" at AskUbuntu may focus on the kernel PPA, but here may focus on downloading the sources from kernel.org and compiling. Should those be merged? Well, not as long as we're going to keep AskUbuntu and Unix & Linux separate.
If you see a multi-post (the same person asking the same question on multiple sites), flag it and leave a comment. The FAQ strongly discourages cross-posting. In these cases, what usually happens is that the question is closed on one site, preferably one where it hasn't been answered yet. Also leave a link on the other site. See also How should questions cross-posted on Ask Ubuntu be handled?
If you see a question that's similar to an existing question on another site, leave a comment, or if you feel like it write an answer that is inspired from that other question (and acknowledge your source if you do so). There is no concept of “cross-site duplicates”. That's assuming the question is on-topic on both sites (e.g. Ubuntu questions may have already been answered on Ubuntu, shell scripting questions may have already been answered on Stack Overflow).