Depending on Tier of tool : Requires Quantity of Ducttape + Tool you wish to repair
Once you have a handy supply of duct tape, you can use it to repair any type of tool.Of course, the more expensive the tool, the more expensive the repair.
Tier 1 tools (Wood and Gold) = 1 tape, Tier 2 tools (Stone) = 2 tapes, Tier 3 tools (Iron) = 4 tapes, Tier 4 tools (Diamond) = 8 tapes.
Whatever state your used Minecraft tool is in, place it in the crafting grid with the appropriate amount of ducttape and it will be fully repaired - as if like new.
For information, Plastic Tools are tier 2.