commit cfe82db5ab0d35832da474014dff4b6dfe33f8b7
Author: Bruce Richardson <bruce.richardson@intel.com>
Date:   Wed Mar 13 10:13:55 2019 +0000

    do not duplicate external dependencies in list
    
    Since the "-l<lib>" flags in the build.ninja file are passed in
    "--start-group"/"--end-group" flags, there should be no need to have any
    library listed twice, even if there are circular dependencies. Therefore we
    can eliminate duplicates. For speed, rather than deduplicating at the end
    of the process, it's faster to not add the duplicate flags in the first
    place.
    
    This should help fix #2150

diff --git a/mesonbuild/build.py b/mesonbuild/build.py
index d456ab86..20f0cdb2 100644
--- a/mesonbuild/build.py
+++ b/mesonbuild/build.py
@@ -1030,8 +1030,9 @@ This will become a hard error in a future Meson release.''')
                 # Deps of deps.
                 self.add_deps(dep.ext_deps)
             elif isinstance(dep, dependencies.Dependency):
-                self.external_deps.append(dep)
-                self.process_sourcelist(dep.get_sources())
+                if dep not in self.external_deps:
+                    self.external_deps.append(dep)
+                    self.process_sourcelist(dep.get_sources())
             elif isinstance(dep, BuildTarget):
                 raise InvalidArguments('''Tried to use a build target as a dependency.
 You probably should put it in link_with instead.''')