The problem with several Climate Change denier arguments is that they are “meta” arguments – philosophical arguments about how people behave, what they intend and how things are done.
One such issue that they take is with the Intergovernmental Panel on Climate Change (IPCC) “consensus” method of operation. They seem to view the IPCC consensus as “blurred lines” – their conclusion is that the IPCC’s unified interpretation of the evidence is suspect.
When the Police want to interview eye-witnesses, and when a judge wants to hear witness evidence, the standard practice is to keep the witnesses apart, so that the lines of evidence can be as independent as possible.
By contrast, in Climate Change Science, there is a certain amount of collaboration between researchers during the course of their work, so you could say that no observations are made independently. However, this should not be labelled as “malicious collusion”, although many Climate Change deniers do do that.