Remove the LICENSE file install from upstream as debian already installs it.
--- siril.orig/meson.build
+++ siril/meson.build
@@ -702,10 +702,9 @@ man_files = [
 ]
 
 install_man(man_files)
-install_data(['AUTHORS', 'LICENSE.md'],
+install_data(['AUTHORS'],
              rename : [
 	       'doc/siril/AUTHORS',
-	       'doc/siril/LICENSE.md',
 	     ],
              install_dir : siril_share)
 
