Thank you for your help MaMaTT8.
On the phone, the Nextcoud setting are well configured : the pictures from the directories (DCIM, screenshots, whatsapp, etc.) are sent to instantupload directories (DCIM, screenshots, whatsapp, etc.).
On the PC browser, instantupload directories are correctly up-to-date. However, there is a folder called Photos that contains an automatic copy of instantupload pictures. Therefore, when I open Photos app on the PC (a Nextcloud app), the picture is displayed twice : 1st time from instantupload directories and 2nd time from Photos directory.
The question is : how to disable the automatic copy from instantupload to Photos directory on Nextcloud area ?
Best regards.