Yeah, sorry, I had your comment open in a tab to answer and then it got lost among the EIGHT BILLION OTHER TABS I've got right now, heh.
If you were crossposting using Semagic, instead of using the Dreamwidth-native crosspost utility, unfortunately there's no way to import your LJ without duplicates resulting: the only way for the importer to know that a particular entry on both sites is the same actual entry is if the entry was generated through the crossposter. (We don't do things like comparing the actual entry content to see if two entries are the same, because text parsing on that level is annoyingly difficult and slow, and the smallest of differences between the two versions could result in entries not being merged when they should be -- or, worse, being merged when they shouldn't be.) The same thing applies if you were manually copying and pasting your entries into both sites' update pages.
My best suggestion is to create a separate account (something like stasia_from_lj or something) and import the current state of your LJ into it -- it might be annoying to have to look in two places for something you remember, but I'd personally find it less annoying than having two copies of all the entries from 2009-2017.
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If you were crossposting using Semagic, instead of using the Dreamwidth-native crosspost utility, unfortunately there's no way to import your LJ without duplicates resulting: the only way for the importer to know that a particular entry on both sites is the same actual entry is if the entry was generated through the crossposter. (We don't do things like comparing the actual entry content to see if two entries are the same, because text parsing on that level is annoyingly difficult and slow, and the smallest of differences between the two versions could result in entries not being merged when they should be -- or, worse, being merged when they shouldn't be.) The same thing applies if you were manually copying and pasting your entries into both sites' update pages.
My best suggestion is to create a separate account (something like stasia_from_lj or something) and import the current state of your LJ into it -- it might be annoying to have to look in two places for something you remember, but I'd personally find it less annoying than having two copies of all the entries from 2009-2017.