diff --git a/core/src/main/assets/messages/scenes/scenes.properties b/core/src/main/assets/messages/scenes/scenes.properties
index 2fe2aa34f..275b0dee7 100644
--- a/core/src/main/assets/messages/scenes/scenes.properties
+++ b/core/src/main/assets/messages/scenes/scenes.properties
@@ -49,6 +49,7 @@ scenes.gamescene.examine=Examine
 scenes.heroselectscene.title=Choose Your Hero
 scenes.heroselectscene.custom_seed_title=Enter a Custom Seed
 scenes.heroselectscene.custom_seed_desc=The same seed and game version always generate the same dungeon! Seeds are nine uppercase letters (e.g. ABC-DEF-GHI), but you can enter anything you want and the game will convert it. _Games with a custom seed cannot earn badges or appear in rankings._
+scenes.heroselectscene.custom_seed_duplicate=You already have a regular game in progress with that seed. You must end that game before using that custom seed.
 scenes.heroselectscene.custom_seed_set=Set
 scenes.heroselectscene.custom_seed_clear=Clear
 
diff --git a/core/src/main/java/com/shatteredpixel/shatteredpixeldungeon/scenes/HeroSelectScene.java b/core/src/main/java/com/shatteredpixel/shatteredpixeldungeon/scenes/HeroSelectScene.java
index 241fb21ed..01374c73c 100644
--- a/core/src/main/java/com/shatteredpixel/shatteredpixeldungeon/scenes/HeroSelectScene.java
+++ b/core/src/main/java/com/shatteredpixel/shatteredpixeldungeon/scenes/HeroSelectScene.java
@@ -211,7 +211,18 @@ public class HeroSelectScene extends PixelScene {
 					public void onSelect(boolean positive, String text) {
 						text = DungeonSeed.formatText(text);
 						long seed = DungeonSeed.convertFromText(text);
+
 						if (positive && seed != -1){
+
+							for (GamesInProgress.Info info : GamesInProgress.checkAll()){
+								if (info.customSeed.isEmpty() && info.seed == seed){
+									SPDSettings.customSeed("");
+									icon.resetColor();
+									ShatteredPixelDungeon.scene().addToFront(new WndMessage(Messages.get(HeroSelectScene.class, "custom_seed_duplicate")));
+									return;
+								}
+							}
+
 							SPDSettings.customSeed(text);
 							icon.hardlight(1f, 1.5f, 0.67f);
 						} else {