diff --git a/src/github.com/matrix-org/dendrite/roomserver/input/events.go b/src/github.com/matrix-org/dendrite/roomserver/input/events.go
index 5fd45ccca..ed7c1f486 100644
--- a/src/github.com/matrix-org/dendrite/roomserver/input/events.go
+++ b/src/github.com/matrix-org/dendrite/roomserver/input/events.go
@@ -32,6 +32,7 @@ type RoomEventDatabase interface {
 	StoreEvent(
 		ctx context.Context,
 		event gomatrixserverlib.Event,
+		txnAndDeviceID *api.TransactionID,
 		authEventNIDs []types.EventNID,
 	) (types.RoomNID, types.StateAtEvent, error)
 	// Look up the state entries for a list of string event IDs
@@ -61,6 +62,13 @@ type RoomEventDatabase interface {
 	MembershipUpdater(
 		ctx context.Context, roomID, targerUserID string,
 	) (types.MembershipUpdater, error)
+	// Look up event ID by transaction's info.
+	// This is used to determine if the room event is processed/processing already.
+	// Returns an empty string if no such event exists.
+	GetTransactionEventID(
+		ctx context.Context, transactionID string,
+		deviceID string, userID string,
+	) (string, error)
 }
 
 // OutputRoomEventWriter has the APIs needed to write an event to the output logs.
@@ -89,8 +97,23 @@ func processRoomEvent(
 		return err
 	}
 
+	if input.TransactionID != nil {
+		var eventID string
+		tdID := input.TransactionID
+		eventID, err = db.GetTransactionEventID(
+			ctx, tdID.TransactionID, tdID.DeviceID, input.Event.Sender(),
+		)
+		if err != nil {
+			return err
+		}
+
+		if eventID != "" {
+			return nil
+		}
+	}
+
 	// Store the event
-	roomNID, stateAtEvent, err := db.StoreEvent(ctx, event, authEventNIDs)
+	roomNID, stateAtEvent, err := db.StoreEvent(ctx, event, input.TransactionID, authEventNIDs)
 	if err != nil {
 		return err
 	}
diff --git a/src/github.com/matrix-org/dendrite/roomserver/storage/sql.go b/src/github.com/matrix-org/dendrite/roomserver/storage/sql.go
index a24dbb1d3..05efa8dd4 100644
--- a/src/github.com/matrix-org/dendrite/roomserver/storage/sql.go
+++ b/src/github.com/matrix-org/dendrite/roomserver/storage/sql.go
@@ -30,6 +30,7 @@ type statements struct {
 	roomAliasesStatements
 	inviteStatements
 	membershipStatements
+	transactionStatements
 }
 
 func (s *statements) prepare(db *sql.DB) error {
@@ -47,6 +48,7 @@ func (s *statements) prepare(db *sql.DB) error {
 		s.roomAliasesStatements.prepare,
 		s.inviteStatements.prepare,
 		s.membershipStatements.prepare,
+		s.transactionStatements.prepare,
 	} {
 		if err = prepare(db); err != nil {
 			return err
diff --git a/src/github.com/matrix-org/dendrite/roomserver/storage/storage.go b/src/github.com/matrix-org/dendrite/roomserver/storage/storage.go
index b94036c9b..cd434da0c 100644
--- a/src/github.com/matrix-org/dendrite/roomserver/storage/storage.go
+++ b/src/github.com/matrix-org/dendrite/roomserver/storage/storage.go
@@ -20,6 +20,7 @@ import (
 
 	// Import the postgres database driver.
 	_ "github.com/lib/pq"
+	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/roomserver/types"
 	"github.com/matrix-org/gomatrixserverlib"
 )
@@ -45,7 +46,8 @@ func Open(dataSourceName string) (*Database, error) {
 
 // StoreEvent implements input.EventDatabase
 func (d *Database) StoreEvent(
-	ctx context.Context, event gomatrixserverlib.Event, authEventNIDs []types.EventNID,
+	ctx context.Context, event gomatrixserverlib.Event,
+	txnAndDeviceID *api.TransactionID, authEventNIDs []types.EventNID,
 ) (types.RoomNID, types.StateAtEvent, error) {
 	var (
 		roomNID          types.RoomNID
@@ -56,6 +58,15 @@ func (d *Database) StoreEvent(
 		err              error
 	)
 
+	if txnAndDeviceID != nil {
+		if err = d.statements.insertTransaction(
+			ctx, txnAndDeviceID.TransactionID,
+			txnAndDeviceID.DeviceID, event.Sender(), event.EventID(),
+		); err != nil {
+			return 0, types.StateAtEvent{}, err
+		}
+	}
+
 	if roomNID, err = d.assignRoomNID(ctx, nil, event.RoomID()); err != nil {
 		return 0, types.StateAtEvent{}, err
 	}
@@ -308,6 +319,18 @@ func (d *Database) GetLatestEventsForUpdate(
 	}, nil
 }
 
+// GetTransactionEventID implements input.EventDatabase
+func (d *Database) GetTransactionEventID(
+	ctx context.Context, transactionID string,
+	deviceID string, userID string,
+) (string, error) {
+	eventID, err := d.statements.selectTransactionEventID(ctx, transactionID, deviceID, userID)
+	if err == sql.ErrNoRows {
+		return "", nil
+	}
+	return eventID, err
+}
+
 type roomRecentEventsUpdater struct {
 	transaction
 	d                       *Database
diff --git a/src/github.com/matrix-org/dendrite/roomserver/storage/transactions_table.go b/src/github.com/matrix-org/dendrite/roomserver/storage/transactions_table.go
new file mode 100644
index 000000000..9faaae654
--- /dev/null
+++ b/src/github.com/matrix-org/dendrite/roomserver/storage/transactions_table.go
@@ -0,0 +1,88 @@
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package storage
+
+import (
+	"context"
+	"database/sql"
+)
+
+const transactionsSchema = `
+-- The transactions table holds transaction IDs with sender's info and event ID it belongs to.
+-- This table is used by roomserver to prevent reprocessing of events.
+CREATE TABLE IF NOT EXISTS roomserver_transactions (
+	-- The transaction ID of the event.
+	transaction_id TEXT NOT NULL,
+	-- The device ID of the originating transaction.
+	device_id TEXT NOT NULL,
+	-- User ID of the sender who authored the event
+	user_id TEXT NOT NULL,
+	-- Event ID corresponding to the transaction
+	-- Required to return event ID to client on a duplicate request.
+	event_id TEXT NOT NULL,
+	-- A transaction ID is unique for a user and device
+	-- This automatically creates an index.
+	CONSTRAINT roomserver_transaction_unique PRIMARY KEY (transaction_id, device_id, user_id)
+);
+`
+const insertTransactionSQL = "" +
+	"INSERT INTO roomserver_transactions (transaction_id, device_id, user_id, event_id)" +
+	" VALUES ($1, $2, $3, $4)" +
+	" ON CONFLICT ON CONSTRAINT roomserver_transaction_unique" +
+	" DO NOTHING"
+
+const selectTransactionEventIDSQL = "" +
+	"SELECT event_id FROM roomserver_transactions" +
+	" WHERE transaction_id = $1 AND device_id = $2 AND user_id = $3"
+
+type transactionStatements struct {
+	insertTransactionStmt        *sql.Stmt
+	selectTransactionEventIDStmt *sql.Stmt
+}
+
+func (s *transactionStatements) prepare(db *sql.DB) (err error) {
+	_, err = db.Exec(transactionsSchema)
+	if err != nil {
+		return
+	}
+
+	return statementList{
+		{&s.insertTransactionStmt, insertTransactionSQL},
+		{&s.selectTransactionEventIDStmt, selectTransactionEventIDSQL},
+	}.prepare(db)
+}
+
+func (s *transactionStatements) insertTransaction(
+	ctx context.Context,
+	transactionID string,
+	deviceID string,
+	userID string,
+	eventID string,
+) (err error) {
+	_, err = s.insertTransactionStmt.ExecContext(
+		ctx, transactionID, deviceID, userID, eventID,
+	)
+	return
+}
+
+func (s *transactionStatements) selectTransactionEventID(
+	ctx context.Context,
+	transactionID string,
+	deviceID string,
+	userID string,
+) (eventID string, err error) {
+	err = s.selectTransactionEventIDStmt.QueryRowContext(
+		ctx, transactionID, deviceID, userID,
+	).Scan(&eventID)
+	return
+}