How To Find A Guid within an If Statement

Here is the problem. User can come to set and enter in text in an upper and lower text box. After they have done so that text will then be stored in a datatable which will hold a Guid Id, a Title string, and a Body string.

How do I arrange it so that a if the page reloads (meaning the data gets sent to the data base) that if the user changes come text and hits submit again that the table will change accordingly and not make a new row.

I am thinking I will need an if statement to pull this off but I am not sure what to put within the statement. There is the code I have thus far in a .net mvc controller

[HttpPost]
public ActionResult ActName(ModelNameModel item)
{
if( ModelState.IsValid)
{
using (DB.DatabaseName db = new DB.DatabaseName())
{   

DB.Model newRecord = new DB.Model();
newRecord.Title = item.Title;
newRecord.Body = item.Body;
newRecord.Id = Guid.NewGuid();
db.Models.Add(newRecord);
db.SaveChanges();
}     
}
return View(item);
}    

What other if statement can I use to verify the Guid Matches? Thank you!