defs section breaks output file because of missing xlink namespace and duplicated IDs

If I have source files using <use xlink:href="..."/> the output file gets corrupted. It is missing the xlink namespace. Alternatively the xlink:href should be converted into href which is supported in modern browsers.

e.g. the European Flag:

<svg xmlns="http://www.w3.org/2000/svg" width="810" height="540" xmlns:xlink="http://www.w3.org/1999/xlink" viewBox="0 0 810 540">
    <defs>
        <g id="d">
            <g id="b">
                <path id="a" d="M0-1l-.31.95.477.156z"/>
                <use xlink:href="#a" transform="scale(-1 1)"/>
            </g>
            <g id="c">
                <use xlink:href="#b" transform="rotate(72)"/>
                <use xlink:href="#b" transform="rotate(144)"/>
            </g>
            <use xlink:href="#c" transform="scale(-1 1)"/>
        </g>
    </defs>
    <path fill="#039" d="M0 0h810v540H0z"/>
    <g fill="#fc0" transform="matrix(30 0 0 30 405 270)">
        <use y="-6" xlink:href="#d"/>
        <use y="6" xlink:href="#d"/>
        <g id="e">
            <use x="-6" xlink:href="#d"/>
            <use xlink:href="#d" transform="rotate(-144 -2.344 -2.11)"/>
            <use xlink:href="#d" transform="rotate(144 -2.11 -2.344)"/>
            <use xlink:href="#d" transform="rotate(72 -4.663 -2.076)"/>
            <use xlink:href="#d" transform="rotate(72 -5.076 .534)"/>
        </g>
        <use xlink:href="#e" transform="scale(-1 1)"/>
    </g>
</svg>

Resulted in a : <svg xmlns="http://www.w3.org/2000/svg"> [1] missing namespace xlink error

Additionally the referenced IDs within the defs section are not rewritten/renumbered. So if multiple files contain the same ID it will also break the output file:

<defs>
  <g id="a"><path d="M0-1l-.31.95.477.156z"/></g>
  <g id="a"><rect width="5" height="1" y="2" x="0"/></g>
</defs>

[fvsch]

I made a template that fixes this issue by checking if xlink:href occurs in the symbols or defs and outputting a xmlns:xlink attribute on the <svg> element in that case: https://github.com/gradientz/assets-builder/blob/master/assets-builder/tasks/svgsymbols-sprite.svg?short_path=5622458

For avoiding duplicating ids, I use gulp-svgmin to rewrite the ids:

// …
.pipe(svgmin(function(file) {
  var name = path.basename(file.relative, path.extname(file.relative))
  var ids = { cleanupIDs: { minify: true, prefix: 'def-' + name + '-' } }
  return { plugins: [ ids ] }
}))
.pipe(svgsymbols({…}))
// …

[bluemoehre]

@Hiswe Sure. Simply check all IDs which will be added together to defs section. Should be enough I think. Thx man.