Issue parsing duplicate keys

[moorereason]

Describe the bug
Two inputs with duplicate keys give invalid or odd results:

Given a={}\na={}, gotoml-test-decoder returns:

{
  "a": {}
}

Given a=1\na=2, gotoml-test-decoder returns:

Error decoding TOML: toml: expected a to be a table, not a value

To Reproduce
https://play.golang.org/p/Z3TgOrkhs5B

toml.Unmarshal([]byte("a={}\na={}"), &v)

https://play.golang.org/p/J59RodyIGaf

toml.Unmarshal([]byte("a=1\na=2"), &v)

Expected behavior
In both cases, I expected to see an error about duplicate keys in the "root" table.

While we get an error with the second input, the error is confusing.

Versions

• go-toml: 2dbd29a
• go version go1.17.1 linux/amd64

Additional context
Found while doing differential fuzzing against BurnstSushi.

[pelletier]

Should be fixed by 4dff8ea. Filed #668 to provide better errors for duplicates tracking.