Clocks must be defined first so that they can be used by other constraints. The first step of the timing constraint creation flow is to identify where the clocks must be defined and whether they must be defined as a primary clock or a generated clock.
-nameoption), you must verify that the clock name is not already used by another clock constraint or an existing auto-generated clock. The Vivado Design Suite timing engine issues a message when a clock name is used in several clock constraints to warn you that the first clock definition is overridden. When the same clock name is used twice, the first clock definition is lost as well as all constraints referring to that name and entered between the two clock definitions. Xilinx recommends that you avoid overriding clock definitions unless no other constraints are impacted and all timing paths remain constrained.