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Bug report

Describe the bug

When upgrading from 8.12 to 8.13 I noticed our process which generates multiple dzi to the same folder (dzi) had failed creating dzi files.

To Reproduce
When running `

vips dzsave file1 dzi/out1
vips dzsave file2 dzi/out2

or

vips dzsave file1 out1
vips dzsave file2 out2

it returns

glib: .: File exists

or

glib: dzi: File exists

and stops

Expected behavior
I would expect dzsave to still be able to write out .dzi files and subfolders to a folder that already exists, and keep the behaviour from 8.12.

Actual behavior
See error message above.

Screenshots
If applicable, add screenshots to help explain your problem.

Environment

• OS : Centos 7, vips installed from remi - vips.x86_64 0:8.13.0-1.el7.remi and vips-tools.8.13.0-1.el7.remi

Additional context
Normal folder structure for our output, which works for the previous version:

I looked for a workaround looking at the options in the cli, but couldn't find a way to make the cli behave/create the folder structure that we are currently using.

Thanks for the awesome work on libvips!