Multiple alerts for a single issue

While monitoring a single URL and the response time was always above the threshold I got 1 alert every 40 seconds:

Apr 15, 2015 11:31:41 AM Response time was above the threshold (1,000 ms) for 23 seconds (until 11:32 AM). The average response time was 1,484 ms.
Apr 15, 2015 11:31:01 AM Response time was above the threshold (1,000 ms) for 23 seconds (until 11:31 AM). The average response time was 1,464 ms.
Apr 15, 2015 11:30:21 AM Response time was above the threshold (1,000 ms) for 23 seconds (until 11:30 AM). The average response time was 1,458 ms.
Apr 15, 2015 11:29:41 AM Response time was above the threshold (1,000 ms) for 23 seconds (until 11:30 AM). The average response time was 1,439 ms.
Apr 15, 2015 11:29:01 AM Response time was above the threshold (1,000 ms) for 23 seconds (until 11:29 AM). The average response time was 1,461 ms.
Apr 15, 2015 11:28:21 AM Response time was above the threshold (1,000 ms) for 23 seconds (until 11:28 AM). The average response time was 1,507 ms.
Apr 15, 2015 11:27:41 AM Response time was above the threshold (1,000 ms) for 23 seconds (until 11:28 AM). The average response time was 1,442 ms.
Apr 15, 2015 11:27:01 AM Response time was above the threshold (1,000 ms) for 23 seconds (until 11:27 AM). The average response time was 1,465 ms.
Apr 15, 2015 11:26:21 AM Response time was above the threshold (1,000 ms) for 23 seconds (until 11:26 AM). The average response time was 1,421 ms.

It should be a single alert with the full duration.