It is not entirely clear to me what variable you want to merge on.
If I understand correctly, the variable you want to merge on in your first file is "datedelareservation", in the second file this is "arrivee". The problem is that there are several variables with the same date for "arrivee". Is that correct?
If this is indeed the case, there is indeed a problem: spss does not know what temperature you want to merge, if there are several "arrivee" dates.
The only way to solve this is to create a file in which each "arrivee" date only occurs once.
You could do that by the "identify duplicate cases" procedure, by which you identify unique "arrivee" cases, and delete the rest.
Another option could be to restructure the file so that there is a separate variable for each temperature matching to an arrivee date, and merge all of these.
in any case, you have to decide first what "arrivee" temperature you want to merge with the first file.
I see now that there is also a datedelareservation variable in the second file, but the same goes here; just replace the word arrivee in the text above.