SQL & PL/SQL

I have an Interval partition (monthly) table that will keep a history for 6 month. After 6 months I need to roll off the oldest partition and create a new one.

We have a job that drops the oldest partition by using

alter table drop partition for <date> ;

One time because of some server issues the job ran twice and it dropped the oldest partition again. So effectively it dropped 2 partitions.

Question: I was under the impression that "alter table drop partition for('01-APR-2010') " will only drop partition only for that month but looks like its dropping May partition too when run again.

Anyone face this issue before

Thanks