People that tan very easily and rarely burn do seem to have slightly less risks of skin cancer formation than those that easily burn. A tan, though, means that skin damage has been done (the skin gets darker in response to sun damage) and could therefore cause a skin cancer to grow. So, whether you tan easily or not, you are never ‘sun safe’, and will be at greater risk of forming skin cancer by tanning or burning.
Also, ‘tanning slowly’ is not a safe way forward. A tan is a tan, and means damage has been done.
It is thought there is also a genetic factor involved in skin cancer, so some people can get lots of sun without ever getting any problems, whilst others may only need a small amount of sun exposure to cause that ‘spark’ towards skin cancer formation. Unfortunately, you may only know which category you fall into once a skin cancer has formed.