When you connect a belt with specific contents to an object that already contains an input with that same contents, add a merger prior to the receiving object.
For instance, you have iron ingots going into a rods constructor. You add another iron ingots belt into that constructor. Create a merger that accepts both inputs, and outputs to the constructor, moving the belt that was previously going to the constructor to the merger.
When you connect a belt with specific contents to an object that already contains an input with that same contents, add a merger prior to the receiving object.
For instance, you have iron ingots going into a rods constructor. You add another iron ingots belt into that constructor. Create a merger that accepts both inputs, and outputs to the constructor, moving the belt that was previously going to the constructor to the merger.