For the website field HubSpot adds http:// in front of the value I'm passing. We don't use a strict format for website and we have values starting with both http and https in our database. This results in records like 'http://HTTP://UU.NL'. Can we somehow prevent HubSpot from adding anything to the website field or what's the recommend approach?
HubSpot will only add the protocol automatically if it doesn't exist already. In the specific example you included, HubSpot adds an additional protocol because technically the all-capital isn't the same as the lower case version. There's no way to prevent this default behavior, so I'd recommend either including the lower case version of the protocol or eliminating the protocol values from your properties.
Converting all records to lowercase did the trick.