Ignore duplicate paths in "systemctl start".

diff --git a/src/systemctl/systemctl.c b/src/systemctl/systemctl.c
index a4290c4..d3ac3ad 100644
--- a/src/systemctl/systemctl.c
+++ b/src/systemctl/systemctl.c
@@ -1596,8 +1596,11 @@ static int start_unit_one(
 
                 r = set_put(s, p);
                 if (r < 0) {
-                        log_error("Failed to add path to set.");
-                        return r;
+                        free(p);
+                        if (r != -EEXIST) {
+                                log_error("Failed to add path %s to set.", p);
+                                return r;
+                        }
                 }
 
                 p = NULL;