Libuv does not support multiple handles watching the same file
descriptor. That condition is caught by an assert but it's detached
from the call site and therefore not always trivial to track down.
This commit turns cases where we can easily detect duplicates into
runtime `UV_EEXIST` errors. More work is needed to catch _all_ cases.
Partially addresses https://github.com/libuv/libuv/issues/1172.
PR-URL: https://github.com/libuv/libuv/pull/1851
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
2256be01b0