[docgen] export causes duplicate doc generation

[konsumlamm]

If export is used on a public binding (for example a proc), the documentation for it is generated twice.

Example

proc test*() = discard

export test

Current Output

Expected Output

Additional Information

This bug appears in the random module (see #16953 (comment)), where the documentation for initRand(int64) is generated twice.

$ nim -v
Nim Compiler Version 1.5.1 [Linux: amd64]
Compiled at 2021-02-08
Copyright (c) 2006-2021 by Andreas Rumpf

git hash: c548f97241c9f0da8f9442c07b99dd9d6adf225a

[timotheecour]

for motivation (since some would object provided example is contrived), a use case is when you already have an exported overload, and then conditionally export another overload.

relevant past PR: #13123

[ringabout]

fixed by #23074