diff --git a/src/main/kotlin/ru/dbotthepony/mc/otm/menu/MatteryMenu.kt b/src/main/kotlin/ru/dbotthepony/mc/otm/menu/MatteryMenu.kt
index a13963a7a..5c7ed9174 100644
--- a/src/main/kotlin/ru/dbotthepony/mc/otm/menu/MatteryMenu.kt
+++ b/src/main/kotlin/ru/dbotthepony/mc/otm/menu/MatteryMenu.kt
@@ -8,6 +8,7 @@ import it.unimi.dsi.fastutil.ints.IntCollection
 import it.unimi.dsi.fastutil.objects.Reference2ObjectFunction
 import it.unimi.dsi.fastutil.objects.Reference2ObjectOpenHashMap
 import it.unimi.dsi.fastutil.objects.ReferenceArrayList
+import it.unimi.dsi.fastutil.objects.ReferenceOpenHashSet
 import net.minecraft.network.FriendlyByteBuf
 import net.minecraft.network.RegistryFriendlyByteBuf
 import net.minecraft.network.protocol.common.custom.CustomPacketPayload
@@ -385,11 +386,11 @@ abstract class MatteryMenu(
 
 	private val externalSlots = ConditionalSet<Slot>()
 	private val quickMoveMapping = Reference2ObjectOpenHashMap<Slot, ReferenceArrayList<Collection<Slot>>>()
+	private val seenSlots = ReferenceOpenHashSet<Slot>()
 
 	override fun addSlot(pSlot: Slot): Slot {
-		if (pSlot in slots) {
+		if (!seenSlots.add(pSlot))
 			return pSlot
-		}
 
 		if (pSlot is UserFilteredSlot && !pSlot.hasSetFilter) {
 			val container = pSlot.container