The objects of the GTextField class generate 3 different types of events.
CHANGED - this is generated whenever the displayed text is changed
SET - this is generated if the setText() method is called
ENTERED - this is generated when the Enter key is pressed (only for single line textfield) or the textfield loses focus.

The following sketch shows how you can test for the event type so I suggest you try it out first.

1. import guicomponents.*;
2. 
   
3. GTextField txf_1, txf_2;
4. 
   
5. void setup() {
6.   size(300, 200);
7.   txf_1 = new GTextField(this, "Hello ", 10, 10, 160, 20, false);
8.   txf_1.setBorder(0);
9.   txf_1.addEventHandler(this, "handleMyTextFieldEvents");
10.   txf_2 = new GTextField(this, "Goodbye ", 10, 40, 160, 20, false);
11. }
12. 
    
13. public void handleMyTextFieldEvents(GTextField t) {
14.   println("textField");
15.   if (t.eventType == GTextField.ENTERED) {
16.     println("  Event type ENTERED");
17.   }
18.   else if (t.eventType == GTextField.CHANGED) {
19.     println("  Event type CHANGED");
20.   }
21.   else if (t.eventType == GTextField.SET) {
22.     println("  Event type SET");
23.   }
24. }
25. 
    
26. void handleTextFieldEvents(GTextField t) {
27.   println("textField");
28.   if (t.eventType == GTextField.ENTERED) {
29.     println("  Event type ENTERED");
30.   }
31.   else if (t.eventType == GTextField.CHANGED) {
32.     println("  Event type CHANGED");
33.     txf_1.setText(t.getText() + " (Copy)");
34.   }
35. }
36. 
    
37. void draw() {
38.   background(255);
39. }

In your code I suggest that you surround the for loop with an if statement to ignore ENTERED even types.

1. public void handleMyTextFieldEvents(GTextField t) {
2.   if(t.eventType == GTextField.CHANGED) {
3.     for (int i = 0; i < layerItems.size(); i++) {
4.       if (layerItems.get(i).getTextField() == t) {
5.         PApplet.println(t.getText());
6.       }
7.     }
8.   } // end of event type if
   
9. }

HTH

By the way have you tried the GUI Builder tool for G4P?

Great, thanks a lot! That kind of makes sense when you look at it that way. :)