"re-rolling" a re-roll would be expressly forbidden by the rules, as it is in Warhammer 40K. When in doubt I often refer back to the Warhammer 40,000 sixth edition rules as a point of reference. You only ever get ONE re-roll in 40K and the result of the re-roll stands. Under Fate Points, directly under the header "Limits on Assistance" on page 204, the Deathwatch rulebook reads: "Re-roll a failed test once. The results of the re-roll are final." This means you get just ONE re-roll and can't use another Fate Point to re-roll the test if the second roll fails. You eat the result even if it is worse than the first roll. As for how many fate points a character can spend in a turn, the Deathwatch core says on that same page, that spending a Fate Point is a Free Action. On page 236 of the Deathwatch core, it explicitly states "A free action takes only a moment and requires no real effort by the character. Free Actions may be performed in addition to any other Actions on a character's Turn, and there is no formal limit to the number of Free Actions one character can take. The GM should use common sense to set reasonable limits on what can be done in a few seconds." This when combined with the information about fate points tells you explicitly that there is no limit to the number of fate points a character can spend in one given turn or round of combat, but it does explicitly say no more than one re-roll per Test.

Personally I feel that Parry should be an advanced skill, and certain specialties get it and can train it at different degrees for XP as per the game mechanics. Real people would dodge an attack instead of trying to block it most of the time. Dodging is a natural reaction, where Parrying requires some practice. Look online at fencing videos and you will see what I mean. Successfully parrying an incoming attack actually requires a modicum of skill and training to pull off, with hours of practice. It should be a skill for all the previous systems as well (Dark Heresy and Rogue Trader) since inquisition and merchant marine type characters aren't always as well trained in combat as say, a Space Marine would be. Space Marines are well versed in combat, but I feel that Parrying should have been a skill from the start. Why have dodge as a skill that can be improved and not do the same for parrying which actually would require more training and practice to pull off? It makes sense that it should be a skill. The Assault Marine should get it either for free as part of their Specialization, or as a cheap upgradable skill. The rest of the Specializations should get it for the normal costs as a "General Space Marine Advance" on the XP tables, so that all characters could gain it and gain training in it for a fixed XP cost (Since Space Marines are trained in the art of Combat anyway).