Remove the LICENSE file install from upstream as debian already installs it.
--- a/Makefile.am
+++ b/Makefile.am
@@ -21,7 +21,7 @@
 	$(INSTALL_DATA) src/gui/gtk_old.css $(DESTDIR)$(pkgdatadir)
 	$(INSTALL_DATA) src/gui/gtk.css $(DESTDIR)$(pkgdatadir)
 	$(mkinstalldirs) $(DESTDIR)$(docdir)
-	$(INSTALL_DATA) AUTHORS LICENSE.md $(DESTDIR)$(docdir)
+	$(INSTALL_DATA) AUTHORS $(DESTDIR)$(docdir)
 	$(mkinstalldirs) $(DESTDIR)$(mandir)/man1
 	$(INSTALL_DATA) siril.1.gz $(DESTDIR)$(mandir)/man1
 	$(mkinstalldirs) $(DESTDIR)$(pkgdatadir)/scripts/
