Window>Arrange>New Windows for XXXXX.INDD
thanks for your your answer.
It could be I have not expressed myself clearly, sorry for that.
What I want to achieve is that the same file cannot be openend more than once.
As you can see in this screenshot, that it is possible to open a file multiple times (by doubleclick or with File>Open).
That is what I want to avoid.
But maybe I remember wrong and this was never possible.
I can't make that happen here. Are you patched to version 11.2?
your case is a special one.
You opened (and did not save) an older version InDesign file in your new version of InDesign.
Every time you are doing this a new window is showing up. And that's ok, I think.
Be careful what you are doing.
Do not overwrite old version files with a new version of InDesign unless you are absolutely sure what you are doing.
You can't go back other than using the IDML exchange format.
To convert old InDesign version files to newer version files you could use a script by Peter Kahrel:
Read the notes and test very carefully.
Always have a backup of the files at hand, in case something goes wrong.
thank you very much for your hint to files created in older version of InDesign.
This solved my problem. After saving it as a new file in current InDesign file format, this new file could only opened once.
Additionally I understand now, why InDesign only offered a "Save As.." dialog box while trying to save that file. It was a file created in an older version of InDesign and Adobe "expects me" to save it with a new name. This corresponds to your hints to be carefull with trying to overwrite old version files.
Your screenshot from your German version gave me the hints:
1. The * in front of the file name = unsaved version of the document
2. "[umgewandelt]" at the end of the file name = opened from a previous version.
thanks for your answer, too. Maybe it is not of interest any more, but not to leave your question unanswered:
I'm using InDesign CC Version 2015.1 Build 188.8.131.52 x64 on Windows 8.1 Professional 64-bit.
Thank you to all, have a nice day!