Fix existing stacks of lures only applying once

src/modifier/modifier.ts

@@ -342,9 +342,19 @@ export class DoubleBattleChanceBoosterModifier extends LapsingPersistentModifier
     return [ this.battlesLeft ];
   }
 
+  /**
+   * Modifies the chance of a double battle occurring
+   * @param args A single element array containing the double battle chance as a NumberHolder
+   * @returns {boolean} Returns true if the modifier was applied
+   */
   apply(args: any[]): boolean {
+    // Use Math.max to ensure no dividing by 0
+    const chanceMultiplier = 2 * Math.max(1, this.getStackCount());
+
     const doubleBattleChance = args[0] as Utils.NumberHolder;
-    doubleBattleChance.value = Math.ceil(doubleBattleChance.value / 2);
+    // This is divided because the chance is generated as a number from 0 to doubleBattleChance.value using Utils.randSeedInt
+    // A double battle will initiate if the generated number is 0
+    doubleBattleChance.value = Math.ceil(doubleBattleChance.value / chanceMultiplier);
 
     return true;
   }