Added check for multiple overrides in manifes files.

manifest.go

@@ -193,6 +193,9 @@ func fromRawManifest(raw rawManifest) (*Manifest, error) {
 		if err != nil {
 			return nil, err
 		}
+		if _, exists := m.Ovr[name]; exists {
+			return nil, errors.Errorf("multiple overrides specified for %s, can only specify one", name)
+		}
 		m.Ovr[name] = prj
 	}
 

manifest_test.go

@@ -107,6 +107,7 @@ func TestReadManifestErrors(t *testing.T) {
 	}{
 		{"multiple constraints", "manifest/error1.toml"},
 		{"multiple dependencies", "manifest/error2.toml"},
+		{"multiple overrides", "manifest/error3.toml"},
 	}
 
 	for _, tst := range tests {

testdata/manifest/error3.toml

@@ -0,0 +1,9 @@
+ignored = ["github.com/foo/bar"]
+
+[[override]]
+  name = "github.com/golang/dep"
+  branch = "master"
+
+[[override]]
+  name = "github.com/golang/dep"
+  branch = "master"