Online demo does not handle DOCTYPE correctly and gives an error

• Are you running the latest version?
• Have you included sample input, output, error, and expected output?
• Have you checked if you are using correct configuration?
• Did you try online tool?

Description

Steps to reproduce:

1. go to the online tool https://naturalintelligence.github.io/fast-xml-parser/
2. no additional input needed, just click "Parse to JSON"
3. then "Parse to XML"
4. and again "Parse to JSON"
5. it'll print Error: XML declaration allowed only at the start of the document.:2:6

I guess this is due to the { "?xml": "", ... } property, which converts to an invalid and duplicated doctype, resulting in an invalid XML:

<?xml version="1.0"?>
<?xml?>
<!-- ... the rest of the XML is valid -->

If i uncheck the "Ignore attributes" checkbox, then at the 2nd step i'll get a JSON that starts with this

{
    "?xml": {
        "@_version": "1.0"
    },

and when you convert it back to XML then you get a valid, but still duplicated doctype, which results in the same error:

<?xml version="1.0"?>
<?xml version="1.0"?>
<!-- ... the rest of the XML is valid -->

Code

The default XML sample that's in the online tool.

Output

expected data

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Start with the online tool at https://naturalintelligence.github.io/fast-xml-parser/ and reproduce the documented Parse to JSON, Parse to XML, and Parse to JSON sequence using its default XML sample. Done means the round trip no longer reports the XML declaration error or produces a duplicated declaration; no source file or test is named in the issue.