diff --git a/src/github.com/matrix-org/dendrite/clientapi/auth/storage/accounts/storage.go b/src/github.com/matrix-org/dendrite/clientapi/auth/storage/accounts/storage.go
index d5712eb5..e88942e3 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/auth/storage/accounts/storage.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/auth/storage/accounts/storage.go
@@ -118,7 +118,8 @@ func (d *Database) SetDisplayName(
 }
 
 // CreateAccount makes a new account with the given login name and password, and creates an empty profile
-// for this account. If no password is supplied, the account will be a passwordless account.
+// for this account. If no password is supplied, the account will be a passwordless account. If the
+// account already exists, it will return nil, nil.
 func (d *Database) CreateAccount(
 	ctx context.Context, localpart, plaintextPassword string,
 ) (*authtypes.Account, error) {
@@ -127,6 +128,9 @@ func (d *Database) CreateAccount(
 		return nil, err
 	}
 	if err := d.profiles.insertProfile(ctx, localpart); err != nil {
+		if common.IsUniqueConstraintViolationErr(err) {
+			return nil, nil
+		}
 		return nil, err
 	}
 	return d.accounts.insertAccount(ctx, localpart, hash)
diff --git a/src/github.com/matrix-org/dendrite/clientapi/jsonerror/jsonerror.go b/src/github.com/matrix-org/dendrite/clientapi/jsonerror/jsonerror.go
index 07fe9030..1bab645f 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/jsonerror/jsonerror.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/jsonerror/jsonerror.go
@@ -103,6 +103,12 @@ func InvalidUsername(msg string) *MatrixError {
 	return &MatrixError{"M_INVALID_USERNAME", msg}
 }
 
+// UserInUse is an error returned when the client tries to register an
+// username that already exists
+func UserInUse(msg string) *MatrixError {
+	return &MatrixError{"M_USER_IN_USE", msg}
+}
+
 // GuestAccessForbidden is an error which is returned when the client is
 // forbidden from accessing a resource as a guest.
 func GuestAccessForbidden(msg string) *MatrixError {
diff --git a/src/github.com/matrix-org/dendrite/clientapi/routing/register.go b/src/github.com/matrix-org/dendrite/clientapi/routing/register.go
index 7bd5820f..3068f521 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/routing/register.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/routing/register.go
@@ -463,6 +463,11 @@ func completeRegistration(
 			Code: 500,
 			JSON: jsonerror.Unknown("failed to create account: " + err.Error()),
 		}
+	} else if acc == nil {
+		return util.JSONResponse{
+			Code: 400,
+			JSON: jsonerror.UserInUse("Desired user ID is already taken."),
+		}
 	}
 
 	token, err := auth.GenerateAccessToken()
diff --git a/src/github.com/matrix-org/dendrite/cmd/create-account/main.go b/src/github.com/matrix-org/dendrite/cmd/create-account/main.go
index 7914a626..99e9b545 100644
--- a/src/github.com/matrix-org/dendrite/cmd/create-account/main.go
+++ b/src/github.com/matrix-org/dendrite/cmd/create-account/main.go
@@ -69,10 +69,13 @@ func main() {
 		os.Exit(1)
 	}
 
-	_, err = accountDB.CreateAccount(context.Background(), *username, *password)
+	account, err := accountDB.CreateAccount(context.Background(), *username, *password)
 	if err != nil {
 		fmt.Println(err.Error())
 		os.Exit(1)
+	} else if account == nil {
+		fmt.Println("Username already exists")
+		os.Exit(1)
 	}
 
 	deviceDB, err := devices.NewDatabase(*database, serverName)
diff --git a/src/github.com/matrix-org/dendrite/common/sql.go b/src/github.com/matrix-org/dendrite/common/sql.go
index f3de66f1..7ac9ac14 100644
--- a/src/github.com/matrix-org/dendrite/common/sql.go
+++ b/src/github.com/matrix-org/dendrite/common/sql.go
@@ -16,6 +16,8 @@ package common
 
 import (
 	"database/sql"
+
+	"github.com/lib/pq"
 )
 
 // A Transaction is something that can be committed or rolledback.
@@ -66,3 +68,9 @@ func TxStmt(transaction *sql.Tx, statement *sql.Stmt) *sql.Stmt {
 	}
 	return statement
 }
+
+// IsUniqueConstraintViolationErr returns true if the error is a postgresql unique_violation error
+func IsUniqueConstraintViolationErr(err error) bool {
+	pqErr, ok := err.(*pq.Error)
+	return ok && pqErr.Code == "23505"
+}