Personally I prefer migration. I often comment on questions that are relevant to us but get posted at stackoverflow or programmers.se, which often leads to duplication but of course the mods there have the option to migrate, then we can merge the dupes at our end.
Of course, migration from digg, slashdot and friends is not possible because the remote end doesn't understand the protocol. In these cases, we should combine duplication with giving better answers so that the community realises they should come here for security discussion.
One final point: blithely copying any old security question from reddit is not something I'd recommend. Stackexchange works best when we have good quality questions that are consistent with the FAQ guidance, and if you post a random question from the inter webs without understanding its context we may require additional clarification that you can't provide.