asp.net core duplicate entry catch error

I want to prevent duplicate entry on post, which works well when there is no parameter for OnGet. Code below

public void OnGet()
{
}
public async Task OnPostAsync()
{
if (!ModelState.IsValid)
{
return Page();
}
var newWOAL = new WOAssetLocation();
if (await TryUpdateModelAsync(newWOAL, "WOAssetLocation", s => s.AssetLocation))
{
var check = await _context.WOAssetLocations.Where(s => s.AssetLocation == newWOAL.AssetLocation).CountAsync();
if (check > 0)
{
ModelState.AddModelError(string.Empty, "Asset Location already exists");
return Page();
}
if (check == 0)
{
_context.WOAssetLocations.Add(newWOAL);
await _context.SaveChangesAsync();
return RedirectToPage("/WO/Location/Index");
}
}
return null;
}

The duplicate is captured and I have error mesage displayed on page. However, when I have OnGet with parameter is not working

public async Task OnGetAsync(string id)
{
if (id == null)
{
return NotFound();
}
var userId = User.FindFirstValue(ClaimTypes.NameIdentifier);
var idorg = await _context.UsersData.Where(u => u.Id == userId).Select(u => u.OrgID).SingleAsync();
OrgID = idorg;
WOAssetLocation = await _context.WOAssetLocations.Where(m => m.WOALId == id).FirstOrDefaultAsync();
if (WOAssetLocation == null)
{
return NotFound();
}
return Page();
}
public async Task OnPostAsync()
{
if (!ModelState.IsValid)
{
return Page();
}
var userId = User.FindFirstValue(ClaimTypes.NameIdentifier);
var idorg = await _context.UsersData.Where(u => u.Id == userId).Select(u => u.OrgID).SingleAsync();
var newWOAA = new WOAssetAsset();
if (await TryUpdateModelAsync(newWOAA, "WOAssetAsset",
s => s.WOALId,
s => s.OrgID,
s => s.AssetID,
s => s.AssetName,
s => s.AssetManufacturer))
{
var check = await _context.WOAssetAssets.Where(s => s.AssetID == newWOAA.AssetID).CountAsync();
if (check > 0)
{
ModelState.AddModelError(string.Empty, "Asset ID already exists");
return Page();
}
if (check == 0)
{
_context.WOAssetAssets.Add(newWOAA);
await _context.SaveChangesAsync();
return RedirectToPage("/WO/Asset/Index", new { id = newWOAA.WOALId });
}
}
return null;
}

the application is throwing an error message and halt.

What should be the approach?

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