when combining items that have some enchantments identical, the described "synergistic effect" on the wiki does not work. Moreover, it may actually lower an enchantment level of the item if the sacrifice has the same enchantment type with a lower level.

In summary: Eff 3 + Eff3 = Eff3 and Eff4 + Eff3 = Eff3, if the repair item has other enchantments

How to reproduce

Start with these items (these are real items which I have in the server, just with a label added for convenience)

/give @p diamond_pickaxe 1 0 {ench:[{id:34,lvl:3},{id:32,lvl:3}],display:{Name:"A"}}
/give @p diamond_pickaxe 1 0 {ench:[{id:34,lvl:3},{id:32,lvl:4}],display:{Name:"B"}}
/give @p diamond_pickaxe 1 0 {ench:[{id:34,lvl:4},{id:35,lvl:3}],display:{Name:"C"}}
/give @p diamond_pickaxe 1 1400 {ench:[{id:34,lvl:3},{id:35,lvl:3}],display:{Name:"D"}}
/give @p book 1 0 {ench:[{id:34,lvl:3}],display:{Name:"E"}}

Put A and B in an anvil (Or B and A, the result is identical)
Expected:
Unb 3 Eff 3 + Unb 3 Eff 4 = Unb 4 Eff 4
Actual:
Unb 3 Eff 3 + Unb 3 Eff 4 = Unb 3 Eff 4 (just consumes A and XP and does nothing to the pickaxe (B))

Put C and D in an anvil (Or D and C, the result is identical)
Expected:
Unb 3 For 3 + Unb 4 For 3 = Unb 4 For 4
Actual:
Unb 3 For 3 + Unb 4 For 3 = Unb 3 For 3 (Not only consumes C and XP but copies the Fortune level of D to C, overwriting the higher level one. Just to be clear, this also happens when swapping C and D)

Put B and E in an anvil
Expected:
Unb 3 Eff 4 + Unb 3 book = Unb 4 Eff 4
Actual:
Unb 3 Eff 4 + Unb 3 book = Unb 3 Eff 4 (just consumes E and XP and does nothing to the item)

Todo
1. Find out whether the case of A+B and B+E is a bug, or if the wiki is unclear/incorrect/outdated
2. Find out what the deal is with C+D (the most prominent bug)