[New Rule] Duplicate blocks in if/else

[sanxiyn]

Explanation

Inspired by if_same_then_else rule in Rust Clippy. See https://rust-lang.github.io/rust-clippy/master/index.html#if_same_then_else. SIM114(#10) is similar, but it only checks if/elif, not if/else.

Example

# Bad
if a:
    b
else:
    b

# Good
b