The built-in slot type AMAZON.USFIRSTNAME has a very large list of names, and it matches those names fairly consistently. One weakness it has, however, is that it does not handle double names (https://en.wikipedia.org/wiki/Double_name), such as "*********", properly.

Steps to reproduce:
1. Open the developer console
2. Create a skill with an intent that has a slot for USFIRSTNAME
3. Open the test console
4. Launch the skill
5. Attempt to the invoke the intent using the name "Billie Jean"

Expected:
-Slot value sent to skill is "Billie Jean"

Actual:
-Slot value sent to skill is "Jean"

I've tested this with several double names, and in each case it picks one of the two words, but is not consistent about which. In order to make sure this wasn't just an issue of infrequency of double names in the US lexicon, I purposely tested with the most-common male and female double names (Mary Kate, Billie Jean, Jim Bob), knowing that their rate of occurrence would be much higher than the bottom tier of single names (e.g. Maeve) which are adequately supported by the slot.