diff --git a/AfRApay.FTM/Program.cs b/AfRApay.FTM/Program.cs
index 9bbd8bb..5b63610 100644
--- a/AfRApay.FTM/Program.cs
+++ b/AfRApay.FTM/Program.cs
@@ -147,7 +147,10 @@ static async void HandleTap(IsoReader reader, HttpClient httpClient, TerminalSta
         case TerminalState.Debit:
         case TerminalState.Credit:
             var finalAmount = Math.Abs(amount) * (state == TerminalState.Debit ? 1 : -1);
-            await CallGet(httpClient, String.Format("/api/card/transaction?card={0}&amount={1}", Convert.ToHexString(uid), finalAmount));
+            var idempotencyKey = new byte[18];
+            Random.Shared.NextBytes(idempotencyKey);
+            await CallGet(httpClient, String.Format("/api/card/transaction?card={0}&amount={1}&ik={2}",
+                Convert.ToHexString(uid), finalAmount, Convert.ToBase64String(idempotencyKey)));
             break;
         case TerminalState.Link:
             await CallGet(httpClient, String.Format("/api/card/link?card={0}", Convert.ToHexString(uid)));
diff --git a/AfRApay.Web/Backend/Tables/User.cs b/AfRApay.Web/Backend/Tables/User.cs
index 1526b59..e2818c1 100644
--- a/AfRApay.Web/Backend/Tables/User.cs
+++ b/AfRApay.Web/Backend/Tables/User.cs
@@ -6,7 +6,8 @@ namespace AfRApay.Web.Backend.Tables;
 
 [Table(Name = "Users")]
 public class User {
-	[Column(Name = "ID"), PrimaryKey, Identity, NotNull] public int     Id       { get; set; }
-	[Column(Name = "Nickname"), NotNull]                 public string  Nickname { get; set; }
-	[Column(Name = "Balance")]                           public decimal Balance  { get; set; }
+	[Column(Name = "ID"), PrimaryKey, Identity, NotNull] public int     Id                { get; set; }
+	[Column(Name = "Nickname"), NotNull]                 public string  Nickname          { get; set; }
+	[Column(Name = "Balance")]                           public decimal Balance           { get; set; }
+	[Column(Name = "LastIdempotencyKey")]                public string LastIdempotencyKey { get; set; }
 }
diff --git a/AfRApay.Web/Controllers/CardTransaction.cs b/AfRApay.Web/Controllers/CardTransaction.cs
index 0f73b57..15f6a05 100644
--- a/AfRApay.Web/Controllers/CardTransaction.cs
+++ b/AfRApay.Web/Controllers/CardTransaction.cs
@@ -7,18 +7,23 @@ namespace AfRApay.Web.Controllers;
 [ApiController, Route("/api/card/transaction")]
 public class CardTransaction : Controller {
 	[HttpGet]
-	public string Get([FromQuery] string card, [FromQuery] decimal amount) {
+	public string Get([FromQuery] string card, [FromQuery] decimal amount, [FromQuery] string ik) {
 		var db = new Database.DbConn();
 		if (db.Cards.Any(p => p.CardId == card)) {
 			var userId = db.Cards.First(p => p.CardId == card).UserId;
 			var user   = db.Users.First(p => p.Id == userId);
-			
-			if (user.Balance - amount < -50) {
-				return "E:Balance too low!";
+
+			// If an idempotency key is given, disregard duplicate requests.
+			if (ik == "" || ik != user.LastIdempotencyKey) {
+				if (user.Balance - amount < -50) {
+					return "E:Balance too low!";
+				}
+
+				user.Balance -= amount;
+				user.LastIdempotencyKey = ik;
+				db.Update(user);
 			}
-			
-			user.Balance -= amount;
-			db.Update(user);
+
 			return $"S:{user.Nickname}: {user.Balance:N2}";
 		}