Instantiating Too Many Objects

I have a turn based strategy game, and it's based on tile movement. I want to put a marker over the tiles the player can move to. I have an array of all the tiles in the map, and then I have a list of the tiles within the move radius. I have a working method for telling and listing which tiles are within the radius, but when I instantiate the markers, it instantiates way too many. Also, I need to do the foreach loop so that the tiles will be placed above each individual one. Any help will be appreciated. Code:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class MovementController : MonoBehaviour
 {
     public int moveRadius;
 
     public GameObject[] tilesInGame;
 
     public List<GameObject> tilesToMove = new List<GameObject> ();
 
     public GameObject moveTarget;
 
 
     void Start()
     {
         tilesInGame = GameObject.FindGameObjectsWithTag ("Node");
         AssignTilesWithinMove ();
     }
 
     void Update()
     {
         
     }
 
     void AssignTilesWithinMove()
     {
             foreach (GameObject tile in tilesInGame) {
                 if (Vector3.Distance (transform.position, tile.transform.position) < moveRadius) {
                     tilesToMove.Add (tile);
 
                     ShowTarget ();
                 }
             }
     }
 
     public void Move()
     {
         
     }
 
     void ShowTarget()
     {
         Vector3 offset = new Vector3 (0, 1, 0);
 
         foreach (GameObject tile in tilesToMove) {
             GameObject targetMarker = (GameObject)Instantiate (moveTarget, tile.transform.position + offset, Quaternion.Euler(90, 0, 0));
         }
     }
 }