Attaching a listener to the same event more than once will only be called at most once

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The listener instance is used as a path marker for Immutable.Map for optimal deletions (i.e. O(1)). Thus, attaching a listener to the same event more than once will only be called at most once.

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A fix is as follows:

Create ref counter wrapping the listener function. This tracks how many times the listener has been attached to the same event and same keyPath. If ref wrapper exists, increment the counter.

• https://github.com/Dashed/probe/blob/cedf9578652b66b2efb9880e73709682fc50d68c/src/index.js#L119-L121

Locate listener, and decrement ref counter. If counter == 0, delete the path from listeners map.

• https://github.com/Dashed/probe/blob/cedf9578652b66b2efb9880e73709682fc50d68c/src/index.js#L173-L177

Unwrap ref wrapper and call listener up to N times.

• https://github.com/Dashed/probe/blob/cedf9578652b66b2efb9880e73709682fc50d68c/src/index.js#L242-L244