Is this an HTML file you're importing or a TXT file? If it's an HTML file, then it's probably specifying the wrong encoding, or none at all.
If it's a TXT file, Sigil recognizes all UTF variants based on BOM presence, and if none is present, falls back to UTF-8. So if you're using a TXT file with a non-Unicode encoding, convert it to UTF-8/16 first.
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