Teams sometimes have a great night, but it has happened too many times of late, so there's no question that Duke is suffering some real defensive breakdowns.
One important statistic is how well teams are shooting the three against Duke, while Duke has been having some real problems. These are the three-point shooting percentages in the ACC (first Duke, then the opponent)
1) Va Tech 37.5/11.1 (W by 9)
2) FSU 41.7/17.5 (W by 8)
3) GaTech 33.3/58.3 (W by 14)
4) NC State 50.0/21.4 (W by 17)
5) Maryland 48.0/16.7 (W by 41)
6) Wake 18.2/20.0 (L by 2)
7) UVA 33.3/25.0 (W by 25)
8) Clemson 23.1/35.0 (L by 27)
9) Miami 30.8/60.0 (W OT by 3)
10) UNC 33.3/40.0 (L by 14)
11) BC 18.8/44.4 (L by 6)
In games 1-5, Duke was clearly superior four times and the team went 5-0. The only exception was GaTech, who shot well in the second half (6/9) in an effort to catch up, but they only shot 3/11 from two-point range and it simply wasn't enough.
The shooting in the Duke-Wake game was pretty even and the game went down to the wire. Duke was better against UVA and won, but then things go south.
Duke was worse in all areas against Clemson and got blown out. Against Miami, McClinton and Dews went 8/10, which is unacceptable (although we have to allow for the fact that McClinton made some unstoppable shots). Duke fortunately won in overtime on pure effort (points off turnovers 24/12 and second chance points 19/10).
Duke was worse against UNC and lost, but the real difference was in the second half when Duke shot 2/15 (13.3%). Against BC Duke shot 3/16, but while Singler shot 3/6 the rest of the team was 0/10. Meanwhile BC hit 44.4.
If we look at it in two-point terms, then this means that the last four teams have shot the equivalent of 52.5%, 90%, 60%, and 66.6% from three-point range, so I would say that Duke has not played good defense against some talented teams.