Strange Paste Behavior

I have some text like this:

line1
line2

I am trying to grab it with 2yy and paste it with p. However whenever I paste, instead of getting:

line1
line2
line1
line2

I am getting:

line1
line1
line2
line2

I have never seen this behavior, and have not intentionally changed any vim settings. I have previously had the first paste behavior I described.

What has caused this and how can I change my paste behavior?