If both are present but different the unprefixed version should be favoured. That seems uncontroversial & not complex to implement.

Sending two headers seems fine in most cases.

These are certainly downsides but hardly dealbreakers. On the other side, not prefixing has its own pros & cons, which seem more difficult to work around:

1. The obvious clash issue. If two pieces of software implement entirely different X-Value: headers, the standardisation effort clarifies the signal in the form of an unprefixed version. If both competing software applications start out unprefixed, the signal will always be ambiguous.

2. Implementation changes. If any lessons are learnt during initial use of a prefixed header, these can be applied by standardising on a slightly improved unprefixed version.