[[ is a much more versatile version of [ in bash.
For example, Using the [[ ... ]] test construct, rather than [ ... ] can prevent many logic errors in scripts. For example, the &&, ||, <, and > operators work within a [[ ]] test, despite giving an error within a [ ] construct.
also, their implementations are different
-bash-3.2$ type [
[ is a shell builtin
-bash-3.2$ type [[
[[ is a shell keyword
Refer here for a good explanation
