diff --git a/src/main/java/io/zold/api/WalletsIn.java b/src/main/java/io/zold/api/WalletsIn.java
index 8e32d6e..4ce5351 100644
--- a/src/main/java/io/zold/api/WalletsIn.java
+++ b/src/main/java/io/zold/api/WalletsIn.java
@@ -36,11 +36,12 @@
 import org.cactoos.iterable.Mapped;
 import org.cactoos.scalar.IoCheckedScalar;
 import org.cactoos.scalar.SolidScalar;
+import org.cactoos.text.FormattedText;
 import org.cactoos.text.JoinedText;
+import org.cactoos.text.UncheckedText;
 
 /**
  * Wallets in path.
- *
  * @since 0.1
  * @checkstyle ClassDataAbstractionCoupling (2 lines)
  */
@@ -61,6 +62,11 @@ public final class WalletsIn implements Wallets {
      */
     private final String ext;
 
+    /**
+     * Randomizer.
+     */
+    private final Random random;
+
     /**
      * Ctor.
      * @param pth Path with wallets
@@ -68,26 +74,43 @@ public final class WalletsIn implements Wallets {
     public WalletsIn(final Path pth) {
         this(
             () -> pth,
-            "z"
+            "z",
+            new Random()
+        );
+    }
+
+    /**
+     * Ctor.
+     * @param pth Path with wallets
+     * @param random Randomizer
+     */
+    public WalletsIn(final Path pth, final Random random) {
+        this(
+            () -> pth,
+            "z",
+            random
         );
     }
 
     /**
      * Ctor.
      * @param pth Path with wallets
+     * @param random Randomizer
      * @param ext Wallets file extension
      */
-    public WalletsIn(final Scalar<Path> pth, final String ext) {
+    public WalletsIn(final Scalar<Path> pth, final String ext,
+        final Random random) {
         this.path = new IoCheckedScalar<>(
             new SolidScalar<>(pth)
         );
         this.filter = new IoCheckedFunc<Path, Boolean>(
             (file) -> file.toFile().isFile()
                 && FileSystems.getDefault()
-                    .getPathMatcher(String.format("glob:**.%s", ext))
-                    .matches(file)
+                .getPathMatcher(String.format("glob:**.%s", ext))
+                .matches(file)
         );
         this.ext = ext;
+        this.random = random;
     }
 
     // @todo #12:30min Create the new wallet in the path with all wallets.
@@ -98,10 +121,20 @@ public Wallet create() throws IOException {
         final Path wpth = this.path.value().resolve(
             new JoinedText(
                 ".",
-                Long.toHexString(new Random().nextLong()),
+                Long.toHexString(this.random.nextLong()),
                 this.ext
             ).asString()
         );
+        if (wpth.toFile().exists()) {
+            throw new IOException(
+                new UncheckedText(
+                    new FormattedText(
+                        "Wallet in path %s already exists",
+                        wpth.toUri().getPath()
+                    )
+                ).asString()
+            );
+        }
         Files.createFile(wpth);
         return new Wallet.File(wpth);
     }
diff --git a/src/test/java/io/zold/api/WalletsInTest.java b/src/test/java/io/zold/api/WalletsInTest.java
index 826daf1..5e53c4e 100644
--- a/src/test/java/io/zold/api/WalletsInTest.java
+++ b/src/test/java/io/zold/api/WalletsInTest.java
@@ -26,10 +26,10 @@
 import java.io.IOException;
 import java.nio.file.Path;
 import java.nio.file.Paths;
+import java.util.Random;
 import org.hamcrest.MatcherAssert;
 import org.hamcrest.collection.IsIterableWithSize;
 import org.hamcrest.core.IsEqual;
-import org.junit.Ignore;
 import org.junit.Rule;
 import org.junit.Test;
 import org.junit.rules.ExpectedException;
@@ -81,19 +81,43 @@ public void createsWalletInFolder() throws IOException {
         );
     }
 
-    // @todo #12:30min Ensure creating a new wallet in a path where a
-    //  wallet with the same id throws an exception and does not
-    //  overwrite the existing one. This could be done using by
-    //  using a Random object with a similar seed in WalletsIn
-    //  and creating a wallet twice for example. When it is done
-    //  enable the test.
     @Test
-    @Ignore("see todo above")
-    public void doesNotOverwriteExistingWallet() throws IOException {
+    public void doesNotOverwriteExistingWallet() throws Exception {
         final Path path = this.folder.newFolder().toPath();
-        new WalletsIn(path).create();
+        final Random random = new FkRandom(16725L);
+        new WalletsIn(path, random).create();
         this.thrown.expect(IOException.class);
-        this.thrown.expectMessage("duplicate");
-        new WalletsIn(path).create();
+        this.thrown.expectMessage("already exists");
+        new WalletsIn(path, random).create();
+    }
+
+    /**
+     * Fake randomizer that returns the same value each time.
+     */
+    private static class FkRandom extends Random {
+
+        /**
+         * Serial version.
+         */
+        private static final long serialVersionUID = 2905348968220129619L;
+
+        /**
+         * Value that represents a random number.
+         */
+        private final long value;
+
+        /**
+         * Ctor.
+         * @param val Value that represents a random number.
+         */
+        FkRandom(final long val) {
+            super();
+            this.value = val;
+        }
+
+        @Override
+        public long nextLong() {
+            return this.value;
+        }
     }
 }