diff --git a/src/game/Game.test.ts b/src/game/Game.test.ts
index 8a0c7fb..3b7530a 100644
--- a/src/game/Game.test.ts
+++ b/src/game/Game.test.ts
@@ -21,7 +21,7 @@ describe("GameState", () => {
 
     test("getting cell works", () => {
         const game = new Game(threeByThreeMatrix, [["AA"]], unrestrictedBuffer, unlimitedTime);
-        expect(game.getCell(0, 2)).toEqual("02")
+        expect(game.getCell(0, 2)).toEqual({ value: "02", isUsed: false })
     });
 
     describe("picking", () => {
@@ -34,11 +34,18 @@ describe("GameState", () => {
             const game = new Game(threeByThreeMatrix, [["AA"]], unrestrictedBuffer, unlimitedTime);
             game.pick(0, 0);
             expect(game.state).toEqual({selectionMode: SelectionMode.RowPick, column: 0});
+            expect(game.getCell(0, 0).isUsed).toEqual(true);
             expect(() => game.pick(0, 2)).toThrow();
             game.pick(2, 0);
             expect(game.state).toEqual({selectionMode: SelectionMode.ColumnPick, row: 2});
         });
 
+        test("cannot pick cell twice", () => {
+            const game = new Game(threeByThreeMatrix, [["AA"]], unrestrictedBuffer, unlimitedTime);
+            game.pick(0, 0);
+            expect(() => game.pick(0, 0)).toThrow();
+        });
+
         test("picking outside of range fails", () => {
             const game = new Game(threeByThreeMatrix, [], unrestrictedBuffer, unlimitedTime);
             expect(() => game.pick(-1, 0)).toThrow();
@@ -52,7 +59,11 @@ describe("GameState", () => {
             const game = new Game(threeByThreeMatrix, [simpleSequence], unrestrictedBuffer, unlimitedTime);
             expect(game.getSequences()).toEqual([{sequence: simpleSequence, numberOfFulfilled: 0}])
             game.pick(0, 0);
-            expect(game.buffer).toEqual(["00"]);
+            expect(game.buffer).toEqual([{
+                positionInMatrixRow: 0,
+                positionInMatrixColumn: 0,
+                value: "00"
+            }]);
             expect(game.getSequences()).toEqual([{sequence: simpleSequence, numberOfFulfilled: 1}])
         });
 
diff --git a/src/game/Game.ts b/src/game/Game.ts
index f6fda32..493f3ce 100644
--- a/src/game/Game.ts
+++ b/src/game/Game.ts
@@ -76,10 +76,21 @@ interface Sequence {
     numberOfFulfilled: number
 }
 
+interface Buffer {
+    value: string
+    positionInMatrixRow: number
+    positionInMatrixColumn: number
+}
+
+interface Cell {
+    value: string
+    isUsed: boolean
+}
+
 export class Game {
     state: State = {selectionMode: SelectionMode.FreePick}
     public readonly size: number
-    public readonly buffer: string[] = []
+    public readonly buffer: Buffer[] = []
     private readonly timeoutInterval: ReturnType<typeof setTimeout>
     private readonly startTimeTimeStamp: number
     private endTimestamp: number | null = null
@@ -106,8 +117,14 @@ export class Game {
         return this.timeout - (Date.now() - this.startTimeTimeStamp)
     }
 
-    getCell(row: number, column: number): string {
-        return this.matrix[row + column * this.size]
+    getCell(row: number, column: number): Cell {
+        return {
+            value: this.matrix[row + column * this.size],
+            isUsed: this.buffer.some(x =>
+                x.positionInMatrixRow == row &&
+                x.positionInMatrixColumn == column
+                ),
+        }
     }
 
     getSequences(): Sequence[] {
@@ -116,7 +133,7 @@ export class Game {
             for (let i = 0; i < this.buffer.length; ++i) {
                 let prefixLength = 0;
                 for (let j = 0; j < Math.min(sequence.length, this.buffer.length - i); ++j) {
-                    if (this.buffer[i + j] != sequence[j]) {
+                    if (this.buffer[i + j].value != sequence[j]) {
                         // abort sequence
                         prefixLength = 0
                         break;
@@ -160,7 +177,16 @@ export class Game {
             Won: () => {throw new IllegalMoveError()},
             Lost: () => {throw new IllegalMoveError()},
             InProgress: (selectionMode) => {
-                this.buffer.push(this.getCell(row, column))
+                const cell = this.getCell(row, column)
+                if (cell.isUsed) {
+                    throw new IllegalMoveError()
+                }
+
+                this.buffer.push({
+                    value: cell.value,
+                    positionInMatrixRow: row,
+                    positionInMatrixColumn: column,
+                })
 
                 matchSelectionState({
                     Free: () => {