diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index 3b8ed07a83a..141f6c4b628 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -1559,8 +1559,24 @@ def _set_compiler(self, compiler):
     def _add_dependency(self, spec, deptypes):
         """Called by the parser to add another spec as a dependency."""
         if spec.name in self._dependencies:
-            raise DuplicateDependencyError("Cannot depend on '%s' twice" % spec)
+            # allow redundant compatible dependency specifications
+            # depspec equality checks by name, so we need to check components
+            # separately to test whether the specs are identical
+            orig = self._dependencies[spec.name]
+            for dspec in orig:
+                if deptypes == dspec.deptypes:
+                    try:
+                        dspec.spec.constrain(spec)
+                        return
+                    except spack.error.UnsatisfiableSpecError:
+                        raise DuplicateDependencyError(
+                            "Cannot depend on incompatible specs '%s' and '%s'"
+                            % (dspec.spec, spec)
+                        )
+            else:
+                raise DuplicateDependencyError("Cannot depend on '%s' twice" % spec)
 
+        # create an edge and add to parent and child
         self.add_dependency_edge(spec, deptypes)
 
     def add_dependency_edge(self, dependency_spec, deptype):
diff --git a/lib/spack/spack/test/spec_syntax.py b/lib/spack/spack/test/spec_syntax.py
index ab560bed084..cf1ce971d01 100644
--- a/lib/spack/spack/test/spec_syntax.py
+++ b/lib/spack/spack/test/spec_syntax.py
@@ -293,6 +293,12 @@ def test_canonicalize(self):
 
         self.check_parse("x ^y", "x@: ^y@:")
 
+    def test_parse_redundant_deps(self):
+        self.check_parse("x ^y@foo", "x ^y@foo ^y@foo")
+        self.check_parse("x ^y@foo+bar", "x ^y@foo ^y+bar")
+        self.check_parse("x ^y@foo+bar", "x ^y@foo+bar ^y")
+        self.check_parse("x ^y@foo+bar", "x ^y ^y@foo+bar")
+
     def test_parse_errors(self):
         errors = ["x@@1.2", "x ^y@@1.2", "x@1.2::", "x::"]
         self._check_raises(SpecParseError, errors)
@@ -481,7 +487,7 @@ def test_multiple_versions(self):
         self._check_raises(MultipleVersionError, multiples)
 
     def test_duplicate_dependency(self):
-        self._check_raises(DuplicateDependencyError, ["x ^y ^y"])
+        self._check_raises(DuplicateDependencyError, ["x ^y@1 ^y@2"])
 
     def test_duplicate_compiler(self):
         duplicates = [