GameObject overlapping on random spawn

Hi,

I am making a side scrolling game in which I randomly generate positions of mountains and other things and then instantiate the object at the position.

I have spent an entire day trying different methods and searching forums and Google for a solution. Every time it looks like it works and then after a couple of instantiations, there is overlapping again. I am completely confused at why nothing has worked. I have tried renderers with bounds, physics, raycasting, and collision detection. Nothing has worked so far.

Here is the mountain spawning method:

  /// <summary>
     /// This method spawns mountains to the map only if they are further right than the camera
     /// </summary>
     void SpawnMountains()
     {
         //Keep spawning mountains until the maximum number has been reached
         while (currentMountains < maxMountains)
         {
             //Get a position for the mountain so we can use it
             GetMountainPosition();
             //If that position is already taken, roll again
             if (mountainPositions.Contains(mountainPosition))
             {
                 GetMountainPosition();
             }           
             else
             {                
                 //Make sure the camera is ahead of the spawn position
                 if (Camera.main.transform.position.x + 10 < mountainPosition.x)
                 {
                     //Create the mountain
                     Instantiate(mountain, mountainPosition, Quaternion.identity);                    
                 }
                 //Check to make sure there are no overlaps 
                 /*
                  * I have tried so many different combinations of things
                  *   to get this to work it works for a while and then they overlap
                  */               
                 if (Physics.CheckSphere(mountainPosition, 7.5f))
                 {
                     Debug.Log("Moving a mountain!");
                     //Get rid of that mountain to prevent it from causing problems 
                     // overlapping another mountain
                     Destroy(mountain);
                     //Roll again
                     GetMountainPosition();
                     //Make sure that the camera is still to the right of the position 
                     if (Camera.main.transform.position.x + 10 < mountainPosition.x)
                     {
                         Instantiate(mountain, mountainPosition, Quaternion.identity);                       
                     }
                 }
                
                 currentMountains++;
                 //Add the position to the list to prevent a duplicate instantiation 
                 mountainPositions.Add(mountainPosition);
             }
           
         }
     }

     /// <summary>
     /// This method gets a random position for a mountain to spawn
     /// </summary>
     void GetMountainPosition()
     {
         mountainPosition = new Vector2(Random.Range(0, target.transform.position.x), 3.2f);
     }

I know that this code isn't very clean and I'm obviously not doing something correctly, but any help would be appreciated.

Here is a screenshot of what is happening.

Thank you.