Hopefully they are planning a probe already.
Bacteria on earth for example, would it have evolved there too?
If the latter isn't the case, could earth bacteria have deletorious consequences on Martian bacteria?
That rather depends - if panspermia turns out to be correct and they have a similar source, then quite possibly - they'll interact much as any two bacteria species on Earth might.
If they are from independent origins of life, then perhaps not - it's possible that these are silicon-based, for instance, rather than carbon-based, and the likelihood of direct interaction will be extremely limited.
What is inevitable is that the introduction of any new form of life will have an indirect effect through competition for resources - space, raw materials such as water etc.
O.
