Adding snippet of code that will unbind a duplicate keystroke in "controls" menu?

Hey guys,

So I wrote this snippet of code for my key assignment section of my game's settings menu. currently, if I click on a specified button, and then input a keyboard key, they button text changes to the pressed key. I would like an included function that states if (for example) the "A" key is already assigned to action1 and pressed to assign to action2, it will remap to action2 and action1's keybind will be cleared and replaced with "-". Im fairly green at coding so I'm wondering what the snippet of code would be and where it would be placed in the following script.

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine.UI;
4. using UnityEngine;
5.  
6. public class KeyBindScript : MonoBehaviour {
7.  
8.     private Dictionary<string, KeyCode> Keys = new Dictionary<string, KeyCode> ();
9.  
10.     private GameObject currentKey;
11.  
12.     public Text forward, backwards, movleft, movright, leanleft, leanright, run, jump, crouch, interact, attack, clairvoyance, journal, inventory, tasks, watch, quickwheel, pausemenu;  
13.  
14.     void Start ()
15.     {
16.         Keys.Add("Forward", KeyCode.W);
17.         Keys.Add("Backwards", KeyCode.S);
18.         Keys.Add("MovLeft", KeyCode.A);
19.         Keys.Add("MovRight", KeyCode.D);
20.         Keys.Add("LeanLeft", KeyCode.Q);
21.         Keys.Add("LeanRight", KeyCode.E);
22.         Keys.Add("Run", KeyCode.LeftShift);
23.         Keys.Add("Jump", KeyCode.Space);
24.         Keys.Add("Crouch", KeyCode.LeftControl);
25.         Keys.Add("Interact", KeyCode.F);
26.         Keys.Add("Attack", KeyCode.Mouse0);
27.         Keys.Add("Clairvoyance", KeyCode.Mouse1);
28.         Keys.Add("Journal", KeyCode.J);
29.         Keys.Add("Inventory", KeyCode.I);
30.         Keys.Add("Tasks", KeyCode.X);
31.         Keys.Add("Watch", KeyCode.C);
32.         Keys.Add("QuickWheel", KeyCode.Mouse3);
33.         Keys.Add("PauseMenu", KeyCode.Escape);
34.  
35.         forward.text = Keys ["Forward"].ToString ();
36.         backwards.text = Keys ["Backwards"].ToString ();
37.         movleft.text = Keys ["MovLeft"].ToString ();
38.         movright.text = Keys ["MovRight"].ToString ();
39.         leanleft.text = Keys ["LeanLeft"].ToString ();
40.         leanright.text = Keys ["LeanRight"].ToString ();
41.         run.text = Keys ["Run"].ToString ();
42.         jump.text = Keys ["Jump"].ToString ();
43.         crouch.text = Keys ["Crouch"].ToString ();
44.         interact.text = Keys ["Interact"].ToString ();
45.         attack.text = Keys ["Attack"].ToString ();
46.         clairvoyance.text = Keys ["Clairvoyance"].ToString ();
47.         journal.text = Keys ["Journal"].ToString ();
48.         inventory.text = Keys ["Inventory"].ToString ();
49.         tasks.text = Keys ["Tasks"].ToString ();
50.         watch.text = Keys ["Watch"].ToString ();
51.         quickwheel.text = Keys ["QuickWheel"].ToString ();
52.         pausemenu.text = Keys ["PauseMenu"].ToString ();
53.  
54.  
55.     }
56.  
57.     void Update ()
58.     {
59.         if (Input.GetKeyDown(Keys["Forward"]))
60.         {
61.             //Move Forward
62.         }
63.         if (Input.GetKeyDown(Keys["Backwards"]))
64.         {
65.             //Move Backwards
66.         }
67.         if (Input.GetKeyDown(Keys["MovLeft"]))
68.         {
69.             //Move Left
70.         }
71.         if (Input.GetKeyDown(Keys["MovRight"]))
72.         {
73.             //Move Right
74.         }
75.         if (Input.GetKeyDown(Keys["LeanLeft"]))
76.         {
77.             //Lean Left
78.         }
79.         if (Input.GetKeyDown(Keys["LeanRight"]))
80.         {
81.             //Lean Right
82.         }
83.         if (Input.GetKeyDown(Keys["Run"]))
84.         {
85.             //Run
86.         }
87.         if (Input.GetKeyDown(Keys["Jump"]))
88.         {
89.             //Jump
90.         }
91.         if (Input.GetKeyDown(Keys["Crouch"]))
92.         {
93.             //Crouch
94.         }
95.         if (Input.GetKeyDown(Keys["Interact"]))
96.         {
97.             //Interact With Object
98.         }
99.         if (Input.GetKeyDown(Keys["Attack"]))
100.         {
101.             //Attack
102.         }
103.         if (Input.GetKeyDown(Keys["Clairvoyance"]))
104.         {
105.             //Initialize Clairvoyance
106.         }
107.         if (Input.GetKeyDown(Keys["Journal"]))
108.         {
109.             //Open Journal
110.         }
111.         if (Input.GetKeyDown(Keys["Inventory"]))
112.         {
113.             //Open Inventory
114.         }
115.         if (Input.GetKeyDown(Keys["Tasks"]))
116.         {
117.             //Open Task Menu
118.         }
119.         if (Input.GetKeyDown(Keys["Watch"]))
120.         {
121.             //Look At Watch
122.         }
123.         if (Input.GetKeyDown(Keys["QuickWheel"]))
124.         {
125.             //Open Quick Wheel
126.         }
127.         if (Input.GetKeyDown(Keys["PauseMenu"]))
128.         {
129.             //Open Pause Menu
130.         }
131.     }
132.  
133.     void OnGUI()
134.     {
135.         if (currentKey != null) {
136.             Event e = Event.current;
137.             if (e.isKey) {
138.                 Keys [currentKey.name] = e.keyCode;
139.                 currentKey.transform.GetChild(0).GetComponent<Text> ().text = e.keyCode.ToString ();
140.                 currentKey = null;
141.             }
142.         }
143.     }
144.  
145.     public void ChangeKey(GameObject clicked)
146.     {
147.         currentKey = clicked;
148.     }
149. }