This error means that the file has been saved with an error. But it sounds like the error in not fatal, and your data may be retrieved back without any loss.
First: what version of LO do you use? We had fixed some bugs like that already, so chances are that if you’d upgrade, you’d not have this kind of problem in the future (however, the file needs fixing anyway).
Newer versions also have ability to open such files even after the error; but this isn’t perfect, since such repair is very likely to loose part of data.
The proper way to fix this file is to open DOCX as ZIP, extract the word/document.xml from it, find the duplicating w:themeTint and remove the second instance in the same tag, and write the fixed XML back to ZIP; possibly several such tags would need to be corrected. If appropriate, you could also rely on others helping you - you could upload the corrupted file to an online share and post a link here - if it’s not some confidential data, of course.
If the problem is with a recent LO version, you’d possibly want to file a bug report - but it’s only useful if you can provide a test document (opening OK) and exact steps what to do with it to get the faulty result.