Hi Leslie,
Congrats on the guest post, please let us know when it's up!
Yes, if you publish the same post in two places word-for-word it will be duplicate content and won't do you any favors in terms of
SEO - it just lessens the search weight carried by the content, so you might want to just paraphrase and link up to it rather than republish it.
The etiquette aspect, I'd say would be up to the blog you're guest posting for - a quick request for permission to republish will clear that one up
Have fun with the guest post! What is the topic? [or do we have to wait to find out?

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