A double block and bleed is a different valve arangement. It is typically used to prevent backflow in case of some type of equipment failure. For example, assume you have a system feeding acid (through the double block) to a process operation. A power failure may cause the acid flow (and whatever the acid is feeding into) to reverse. The reverse flow may cause damage to other equipment. If you were feeding acid into water, the reverse flow of water into the bulk acid tank may cause the acid tank to explode.
Double block valves are typically equipped with spring to close actuators that close in case of a failure or loss of power. The bleed off between the valves is equipped with a spring to open actuator so that the needle valve drains. The pipe between the double block valves is then depressurized and you have a fool-proof pipeline flow blocking device. Actually double block valves are quite similar to backflow preventer devices.If you have two valves in series, than I would concur with Stanier's comments, it does not make any sense.
There is little value to installing redundant valves. One would be better off economically to buy a better quality valve than buy two valves that they do not trust.

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