With the example bellow, the player who is holding/wearing the helmet rarely gets knocked, while most of the time he/she receives no knockback at all. This is of course not the intended result, as the player should be knocked back 50% as much as normal.
give @p 314 1 0 {AttributeModifiers:[{AttributeName:"generic.knockbackResistance",Name:"generic.knockbackResistance",Amount:0.5,Operation:0,UUIDLeast:894654,UUIDMost:2872}]}
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Yes it is. Please update.

Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases. If this has been done, we can reopen the issue.
Keep in mind that the "Resolved"-Status on this ticket just means "Answered", and that we are waiting for further information on whether this issue still exists or not. We will reopen it as soon as the requested information has been delivered.
Ticket resolved as incomplete, because no answer in a reasonable amount of time (1+ year), if it still happens, please update the ticket.
Reopened and confirmed, though this is fixed in the combat test snapshots. I thought there was another ticket for this, though I could not find it.
Still a concern in 1.7.5 and 08a