In the 'Heavy' Specialization, 'Durable' is listed only once but the description mentions 'by 10 per rank of Durable'. Does this mean that Talents like this one are purchasable more than once? If that be the case, which ones can be purchased more than once and which ones cannot?

The Talent descriptions will list if a Talent is considered ranked.

Other Specializations have the same talent and sometimes more than once.

If you get a second rank in durable from another talent tree you will get the second rank and reduce criticals by 20 and so on, but you can't buy it twice in one talent tree if its not pictured twice.

ed: also the talent description in the rule book tells you if its ranked or not like 2P51 said.