The Sovereign Military Order of Malta has formal relations with over 100 states, but not with various states like the UK and the US. In addition there are (at least) five states that don't recognize the SMOM's passports. The Netherlands being one of them. According to wikipedia, there are two citizens of the SMOM. If they did not have dual citizenship, would there be any legal way for them to visit Amsterdam (outside of a diplomatic visit)?
If they had SMOM citizenship and no other, then as far as the government of the Netherlands is concerned, they are stateless.
The Netherlands is a member of the Schengen travel zone. Entry to Schengen requires a "valid travel document", as recognised by the national government in question. For the Netherlands, an SMOM passport is not a valid travel document. So they could not enter the Netherlands from outside Schengen, unless the Netherlands government decided to make a special exception on political or humanitarian grounds.
Entry to the Netherlands from inside Schengen would be possible. For example they could simply walk across the border from Germany, which is also a member of Schengen and recognises SMOM passports. Obviously their immigration status would present problems if they wished to live and work in the Netherlands, but a brief visit would presumably not be an issue.
It's worth noting that SMOM citizenship is issued only to the three most senior officials of the order. They certainly had some other citizenship before attaining their senior positions in the SMOM, and for exactly this reason it would be very foolish of them to renounce it.