A Chance to Try Again

This semester I asked students in my classes to give presentations on their papers. I've been very generous in grading these presentations. And I realized that part of the reason I was being so generous was because I was only giving them a chance to present once. In the past, when I've had students present I give them a chance to do it twice and I am much more critical in my grading. This made me think that I operate under the following grading policy: Only grade a particular assignment harshly if students had a chance to try that kind of assignment before. So, for example, if you are going to grade papers harshly, then you should have more than one paper due a term (or a draft in which you give them comments) so students can learn from the mistakes they make the first time around. Thoughts? 

[Edited: The original post didn't quite capture what I meant. Thanks to Sarah Paul for pointing this out.]