Generating same random numbers?

My system: OS: Windows 7 RStudio 0.99.465, R 3.2.2

I opened two entities of RStudio and ran the same command which generates random numbers. Surprisingly, the randomly generated numbers from two sessions are identical. Note that I did NOT use set.seed command.

For the 1st and 2nd session, the same command was run before I ran 'rgamma(5,1,1)'. For the 3rd, I ran 'rgamma(5,1,1)' right after RStudio opened.

Is this normal operation of R?