running cargo clippy on a crate that has the clippy plugin enabled errors out due to duplicate lints

Author: oli-obk

src/lib.rs

@@ -12,7 +12,11 @@ pub use clippy_lints::*;
 
 #[plugin_registrar]
 pub fn plugin_registrar(reg: &mut Registry) {
-    register_plugins(reg);
+    if reg.sess.lint_store.borrow().get_lint_groups().iter().any(|&(s, _, _)| s == "clippy") {
+        reg.sess.struct_warn("running cargo clippy on a crate that also imports the clippy plugin").emit();
+    } else {
+        register_plugins(reg);
+    }
 }
 
 // only exists to let the dogfood integration test works.