Remove the LICENSE file install from upstream as debian already installs it.
--- a/Makefile.am
+++ b/Makefile.am
@@ -21,8 +21,7 @@
 	3rdparty/scripts/en/DSLR_Preprocessing_NoFlat_NoDark.ssf \
 	3rdparty/scripts/en/DSLR_Preprocessing_NoFlat.ssf
 
-doc_DATA = AUTHORS \
-	LICENSE.md
+doc_DATA = AUTHORS
 
 man_MANS = siril.1
 
