Any possibility to find out in which catalog a photo is imported already?

You could consider a plug-in that provides a listing of a catalogs content. I have never tried this  so I cannot vouch for any of these.

for example, see:

http://www.beardsworth.co.uk/lightroom/list-view/

https://alloyphoto.com/plugins/export-list/

@dieterl94623937 

Do you need history in your catalogs?

Do you have custom metadata fields in one or all those catalogs?

Two more solutions:

1. Save the following lines to the file "findfile.bat" in your home folder:

@4628292 off
set root=c:\users\john\pictures\lightroom
for /d %%d in ("%root%\*") do (
    for %%c in ("%%d\*.lrcat") do (
        find /c /i "%1" "%%c"
        )
    )

Change the second line to refer to the folder that contains all your catalog folders.  Then to search for a particular file, run this at the command prompt:

findfile "test-pics\aa.raf"

substituting between the double quotes the file or path of the file you're searching for.

Every catalog will be searched for occurrences of that path, and the count of the number of matches will be output after the catalog's path.

This isn't very fast, since the Windows "find" command is pretty slow. But it doesn't require you to do anything special (like build a master catalog).

----------------------------------------------------

2. A variant of C.Cella's suggestion: Build a master catalog of all your pics once, and then incrementally update it whenever you need to search for a file.

a. In each catalog, add a unique keyword with the catalog's name to all of its photos.

b. In each catalog, enable the option Catalog Settings > Metadata > Automatically Write Changes Into XMP.

c. Build a master catalog containing all your pics, being sure to Add rather than Copy or Move the imported photos.

d. In the master catalog, disable the option Catalog Settings > Metadata > Automatically Write Changes Into XMP.

e. In the master catalog, make sure the root folder containing all the catalogs' pics is visible in the Folder panels -- if it isn't, repeatedly right-click a top-level folder and do Show Parent Folder.

f. To incrementally update the master catalog to contain the latest contents of your catalogs:

i. Right-click the root folder and do Synchronize Folder and enable the options Remove Missing Photos From Catalog and Show Import Dialog Before Importing. Make sure to select Add in the Import window.

ii. In the Library Filter bar's Metadata browser, use the column Metadata Status. Select Changed On Disk and Conflict Detected. (You can define a filter preset with these settings.) Select all the displayed files and do the menu command Metadata > Read Metadata From File.

Jepp, thats what I thought already...

But, what if I would import all catalogs into one mastercatalog. Would IN THIS CASE be any possibility to get the information in which of the single-catalog this picture is?

Of course this would not be practical, because I could always use a "master catalog" instead of my many "project catalogs" - about 30 pieces at the moment.

Only one of the reasons why I use several catalogs is the much faster backup, because with well over 1 million photos in total, a single catalog in this sense would also be somewhat unwieldy.

I guess, I simply need more concentration, not to import photos into wrong catalogs... to avoid the problem that I have at the moment (not really a problem, but a uncomfortable situation)  😉

Thanks to all!

This is a good reason to use one catalog. But since you say you need to use many catalogs (I'm skeptical), there is no way to know which catalog(s) a photo is in without doing the searching you describe.

No!

There is no link between catalogs. So, no way for one catalog to indicate or even be aware that a photo has or has not be imported into another catalog.