var dbContext = new ModelRequestContainer();
var decreeNums = personToSave.Select(person => person.DecreeNum) //DecreeNum - key
    .Distinct();
var decreeNumPresentInDb = dbContext.BailiffRequests
    .Where(person => decreeNums.Contains(person.DecreeNum))
    .Select(person => person.DecreeNum);
var uniquePersonToSave = personToSave
    .Where(person => !decreeNumPresentInDb.Contains(person.DecreeNum));
dbContext.BailiffRequests.AddRange(uniquePersonToSave);
dbContext.SaveChanges();