diff --git a/cmd/mothd/issues_test.go b/cmd/mothd/issues_test.go
new file mode 100644
index 0000000..26a6f6d
--- /dev/null
+++ b/cmd/mothd/issues_test.go
@@ -0,0 +1,28 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/spf13/afero"
+)
+
+func TestIssue156(t *testing.T) {
+	puzzles := NewTestMothballs()
+	state := NewTestState()
+	theme := NewTestTheme()
+	server := NewMothServer(Configuration{}, theme, state, puzzles)
+
+	afero.WriteFile(state, "teams/bloop", []byte("bloop: the team"), 0644)
+	state.refresh()
+
+	handler := server.NewHandler("", "bloop")
+	es := handler.ExportState()
+	if _, ok := es.TeamNames["self"]; !ok {
+		t.Fail()
+	}
+
+	err := handler.Register("bloop: the other team")
+	if err != ErrAlreadyRegistered {
+		t.Fail()
+	}
+}
diff --git a/cmd/mothd/server.go b/cmd/mothd/server.go
index 608e6d6..f53ecdf 100644
--- a/cmd/mothd/server.go
+++ b/cmd/mothd/server.go
@@ -184,12 +184,15 @@ func (mh *MothRequestHandler) ExportState() *StateExport {
 	return mh.exportStateIfRegistered(false)
 }
 
-func (mh *MothRequestHandler) exportStateIfRegistered(override bool) *StateExport {
+// Export state, replacing the team ID with "self" if the team is registered.
+//
+// If forceRegistered is true, go ahead and export it anyway
+func (mh *MothRequestHandler) exportStateIfRegistered(forceRegistered bool) *StateExport {
 	export := StateExport{}
 	export.Config = mh.Config
 
 	teamName, err := mh.State.TeamName(mh.teamID)
-	registered := override || mh.Config.Devel || (err == nil)
+	registered := forceRegistered || mh.Config.Devel || (err == nil)
 
 	export.Messages = mh.State.Messages()
 	export.TeamNames = make(map[string]string)
diff --git a/cmd/mothd/state.go b/cmd/mothd/state.go
index d8f09d1..eca4d5b 100644
--- a/cmd/mothd/state.go
+++ b/cmd/mothd/state.go
@@ -141,6 +141,13 @@ func (s *State) TeamName(teamID string) (string, error) {
 // SetTeamName writes out team name.
 // This can only be done once per team.
 func (s *State) SetTeamName(teamID, teamName string) error {
+	s.lock.RLock()
+	_, ok := s.teamNames[teamID]
+	s.lock.RUnlock()
+	if ok {
+		return ErrAlreadyRegistered
+	}
+
 	idsFile, err := s.Open("teamids.txt")
 	if err != nil {
 		return fmt.Errorf("team IDs file does not exist")