#merging-multiple-gitignore-files-without-duplicated-rules.js

function mergeIgnorotronsFiles(...gitignoresContent) {
    return [
        ...new Set(
            gitignoresContent
                .join("\n")
                .split("\n")
                .filter((x) => x.charAt(0) != "#")
                .map((x) => x = x.trim())
                .filter((x) => x)
        )
    ].join("\n");
}

for test example use github's gitignore repo

git clone https://github.com/github/gitignore

as arguments of mergeIgnorotronsFiles function use .gitignore content text

example code sample with node.js combination of C++.gitignore and C.gitignore

//to use it in script this code should be combined with needed build-in module node:fs
/* 
const fs = require('fs');
*/
//first ignore file
let firstGitignore = fs.readFileSync(
    "./gitignore/C.gitignore",
    {encoding:"utf8"}
);
//second ignore file
let secondGitignore = fs.readFileSync(
    "./gitignore/C++.gitignore",
    {encoding:"utf8"}
);
//EXAMPLE OF USE
let fusionOfGitignores = mergeIgnorotronsFiles(
    firstGitignore,
    secondGitignore
);
//you can write it out into file like this
fs.writeFileSync(
    "./.gitignore",
    fusionOfGitignores,
    {encoding:"utf8"}
);

Note, that in linux system files, started with comma (.) are in default hidden, so when you look for it use ls -al