There are quite a few things to take note of.
1) Volume for the second top must be distinctly lower than the first top (if the first top itself was on lower than avg vols than the pattern is suspect But you must also look at the volumes of the bars leading to the top bar)
2) The trough between the 2 tops must be at least at a 10% retracement level. The DT becomes valid only after this trough is broken after retracement from the 2nd top.
3) The time between the two tops should not be less than 2 wks (10 bars)
4) Generally the second top will exceed the first one but if the difference is more than 3% then it generally will not qualify as DT
There is a more minor version of DT - (Trader Vic) 2B top - where the time difference between the 2 tops may only be a few bars, but this is generally seen only in intraday charts.
Regards,
Kalyan.