diff --git a/convex/polar.ts b/convex/polar.ts
index 946f363..7415848 100644
--- a/convex/polar.ts
+++ b/convex/polar.ts
@@ -4,6 +4,11 @@ import { internalMutation, type MutationCtx } from "./_generated/server";
 
 type DbCtx = Pick<MutationCtx, "db">;
 
+type IdempotencyScope =
+  | "topup_paid"
+  | "subscription_activated_cycle"
+  | "subscription_revoked";
+
 type ActivatedArgs = {
   userId: string;
   tier: "starter" | "pro" | "max";
@@ -26,12 +31,21 @@ type TopUpArgs = {
 };
 
 export async function applySubscriptionActivated(ctx: DbCtx, args: ActivatedArgs) {
-  const existing = await ctx.db
+  const existingByPolar = await ctx.db
     .query("subscriptions")
-    .withIndex("by_user", (q) => q.eq("userId", args.userId))
-    .order("desc")
+    .withIndex("by_polar", (q) => q.eq("polarSubscriptionId", args.polarSubscriptionId))
     .first();
 
+  const existingByUser = existingByPolar
+    ? null
+    : await ctx.db
+        .query("subscriptions")
+        .withIndex("by_user", (q) => q.eq("userId", args.userId))
+        .order("desc")
+        .first();
+
+  const existing = existingByPolar ?? existingByUser;
+
   if (existing) {
     await ctx.db.patch(existing._id, {
       tier: args.tier,
@@ -51,6 +65,19 @@ export async function applySubscriptionActivated(ctx: DbCtx, args: ActivatedArgs
     });
   }
 
+  const cycleKey = `polar:subscription_cycle:${args.polarSubscriptionId}:${args.currentPeriodStart}:${args.currentPeriodEnd}`;
+  const isFirstCycleEvent = await registerWebhookEvent(ctx, {
+    provider: "polar",
+    scope: "subscription_activated_cycle",
+    idempotencyKey: cycleKey,
+    userId: args.userId,
+    polarSubscriptionId: args.polarSubscriptionId,
+  });
+
+  if (!isFirstCycleEvent) {
+    return;
+  }
+
   const balance = await ctx.db
     .query("creditBalances")
     .withIndex("by_user", (q) => q.eq("userId", args.userId))
@@ -82,11 +109,22 @@ export async function applySubscriptionActivated(ctx: DbCtx, args: ActivatedArgs
 }
 
 export async function applySubscriptionRevoked(ctx: DbCtx, args: RevokedArgs) {
-  const sub = await ctx.db
-    .query("subscriptions")
-    .withIndex("by_user", (q) => q.eq("userId", args.userId))
-    .order("desc")
-    .first();
+  const subByPolar = args.polarSubscriptionId
+    ? await ctx.db
+        .query("subscriptions")
+        .withIndex("by_polar", (q) => q.eq("polarSubscriptionId", args.polarSubscriptionId))
+        .first()
+    : null;
+
+  const subByUser = subByPolar
+    ? null
+    : await ctx.db
+        .query("subscriptions")
+        .withIndex("by_user", (q) => q.eq("userId", args.userId))
+        .order("desc")
+        .first();
+
+  const sub = subByPolar ?? subByUser;
 
   if (sub) {
     await ctx.db.patch(sub._id, {
@@ -107,6 +145,21 @@ export async function applySubscriptionRevoked(ctx: DbCtx, args: RevokedArgs) {
     });
   }
 
+  const revokedKey = args.polarSubscriptionId
+    ? `polar:subscription_revoked:${args.polarSubscriptionId}`
+    : `polar:subscription_revoked:user:${args.userId}`;
+  const isFirstRevokedEvent = await registerWebhookEvent(ctx, {
+    provider: "polar",
+    scope: "subscription_revoked",
+    idempotencyKey: revokedKey,
+    userId: args.userId,
+    polarSubscriptionId: args.polarSubscriptionId,
+  });
+
+  if (!isFirstRevokedEvent) {
+    return;
+  }
+
   await ctx.db.insert("creditTransactions", {
     userId: args.userId,
     amount: 0,
@@ -119,13 +172,15 @@ export async function applySubscriptionRevoked(ctx: DbCtx, args: RevokedArgs) {
 }
 
 export async function applyTopUpPaid(ctx: DbCtx, args: TopUpArgs) {
-  const duplicate = await ctx.db
-    .query("creditTransactions")
-    .withIndex("by_user", (q) => q.eq("userId", args.userId))
-    .filter((q) => q.eq(q.field("description"), `Top-up order ${args.polarOrderId}`))
-    .first();
+  const isFirstTopUpEvent = await registerWebhookEvent(ctx, {
+    provider: "polar",
+    scope: "topup_paid",
+    idempotencyKey: `polar:order_paid:${args.polarOrderId}`,
+    userId: args.userId,
+    polarOrderId: args.polarOrderId,
+  });
 
-  if (duplicate) {
+  if (!isFirstTopUpEvent) {
     return;
   }
 
@@ -135,7 +190,7 @@ export async function applyTopUpPaid(ctx: DbCtx, args: TopUpArgs) {
     .unique();
 
   if (!balance) {
-    return;
+    throw new Error(`Missing credit balance for user ${args.userId}`);
   }
 
   await ctx.db.patch(balance._id, {
@@ -152,6 +207,41 @@ export async function applyTopUpPaid(ctx: DbCtx, args: TopUpArgs) {
   });
 }
 
+async function registerWebhookEvent(
+  ctx: DbCtx,
+  args: {
+    provider: "polar";
+    scope: IdempotencyScope;
+    idempotencyKey: string;
+    userId: string;
+    polarOrderId?: string;
+    polarSubscriptionId?: string;
+  }
+) {
+  const existing = await ctx.db
+    .query("webhookIdempotencyEvents")
+    .withIndex("by_provider_key", (q) =>
+      q.eq("provider", args.provider).eq("idempotencyKey", args.idempotencyKey)
+    )
+    .first();
+
+  if (existing) {
+    return false;
+  }
+
+  await ctx.db.insert("webhookIdempotencyEvents", {
+    provider: args.provider,
+    scope: args.scope,
+    idempotencyKey: args.idempotencyKey,
+    userId: args.userId,
+    polarOrderId: args.polarOrderId,
+    polarSubscriptionId: args.polarSubscriptionId,
+    createdAt: Date.now(),
+  });
+
+  return true;
+}
+
 export const handleSubscriptionActivated = internalMutation({
   args: {
     userId: v.string(),
diff --git a/convex/schema.ts b/convex/schema.ts
index 5c2ceb7..8360350 100644
--- a/convex/schema.ts
+++ b/convex/schema.ts
@@ -258,6 +258,22 @@ export default defineSchema({
     .index("by_polar", ["polarSubscriptionId"])
     .index("by_lemon_squeezy", ["lemonSqueezySubscriptionId"]),
 
+  webhookIdempotencyEvents: defineTable({
+    provider: v.union(v.literal("polar")),
+    scope: v.union(
+      v.literal("topup_paid"),
+      v.literal("subscription_activated_cycle"),
+      v.literal("subscription_revoked")
+    ),
+    idempotencyKey: v.string(),
+    userId: v.string(),
+    polarOrderId: v.optional(v.string()),
+    polarSubscriptionId: v.optional(v.string()),
+    createdAt: v.number(),
+  })
+    .index("by_provider_key", ["provider", "idempotencyKey"])
+    .index("by_user", ["userId"]),
+
   // ==========================================================================
   // Abuse Prevention
   // ==========================================================================