This getting complicated. The repeated word was not detected. So I try to uninstall LT from LibreOffice. This was impossible. See the screenshot:
As you can see in the screen LibreOffice was in the process removing LT.
This means LT did corrupt one way or the other LibreOffice.
So I uninstalled LibreOffice all the way and re-installed it. Now I have got LibreOffice version 7.0.4.2
I checked again twice the same word, but it was not detected. Now I want to install LanguageTool again but it was already installed (version 5.1.3) This seems the default configuration with LibreOffice.
Just try to use LT, just try to install LT requires much more then I expected.
What can be the next step?