EF Core ‘another instance is already being tracked’

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By default when you retrieve entities they are tracked and since they are tracked you could just call SaveChanges and not call Update. You can also retrieve entities without tracking them by using .AsNoTracking()

calling Update is needed if not tracked already, so if you use AsNoTracking then you do need to use Update before SaveChanges

public IQueryable<Anomaly> GetAll()
{    return _context.Anomalies
    .Include(a => a.Asset)
    .Include(a => a.Level);
}

public async Task<Anomaly> GetAnomaly(int anomalyId, User user)
{
    var anomaly = await GetAll()
        .AsNoTracking()
        .FirstOrDefaultAsync(a => a.Id == anomalyId);

    return anomaly;
}

You can also check if the entity is tracked to know whether to call Update or not:

using (var transaction = _context.Database.BeginTransaction())
{
    try
    {

        bool tracking = _context.ChangeTracker.Entries<Anomaly>().Any(x => x.Entity.Id == anomaly.Id);
        if (!tracking)
        {
            _context.Anomalies.Update(anomaly);
        }

        _context.SaveChanges();

        transaction.Commit();
    }
    catch (Exception ex)
    {
        transaction.Rollback();
        throw;
    }
}

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