The code for SD::add_loader_constraint() doesn't check whether the class is already present in both loaders:

diff --git a/src/share/vm/classfile/loaderConstraints.cpp b/src/share/vm/classfile/loaderConstraints.cpp
--- a/src/share/vm/classfile/loaderConstraints.cpp
+++ b/src/share/vm/classfile/loaderConstraints.cpp
@@ -186,8 +186,9 @@
                                        Klass* klass2, Handle class_loader2) {
    int failure_code = 0; // encode different reasons for failing

- if (klass1 != NULL && klass2 != NULL && klass1 != klass2) {
- failure_code = 1;
+ if (klass1 != NULL && klass2 != NULL) {
+ // Same type already loaded in both places. There is no need for any constraint.
+ failure_code = (klass1 == klass2) ? 0 : 1;
    } else {
      Klass* klass = klass1 != NULL ? klass1 : klass2;