The logic for the third parameter (addDuplicateNumbers) does not respect a falsy value.

addDuplicateNumbers = addDuplicateNumbers ~= nil and addDuplicateNumbers or true

When a value of false is passed the logic is resolved as follows:

1. addDuplicateNumbers ~= nil resolves to true
2. addDuplicateNumbers ~= nil and addDuplicateNumbers is therefore the same as true and false which resolves to false
3. addDuplicateNumbers ~= nil and addDuplicateNumbers or true is therefore the same a false or true, which resolves to true

So a falsy value always coerces to true.

Simple test script:

local t1 = { 1 }
local t2 = { 1 }

local tmerged = lib.table.merge(t1, t2, false)
print("Merged", json.encode(tmerged))
--------
-- Output:
-- Merged   [2]

Proposed solution is to reverse the logic somewhat so it sets the value to true only when nil.

addDuplicateNumbers = addDuplicateNumbers == nil and true or addDuplicateNumbers

Running the same test script:

-- Output:
-- Merged   [1]