ruby remove same value multiple keys in hash

A pair of a hash's key/value pairs, k1=>v1 and k2=>v2, are both to be deleted if (v1-v2).abs <= 50. This includes pairs for which v1 == v2, so we need not consider the latter separately. I would do this by first constructing an array of keys to keep, then create a hash comprised of the corresponding key/value pairs from the original hash.

Code

keys_to_keep = hash.keys -
  hash.sort_by { |_,v| v }
      .each_cons(2)
      .each_with_object([]) {
        |((k1,v1),(k2,v2)),a| a << k1 << k2 if (v1-v2).abs <= 50 }

keys_to_keep.zip(hash.values_at(*keys_to_keep)).to_h

Explanation

hash = {"a"=>100,"b"=>200,"c"=>100,"d"=>120}

Sort by hash values:

b = hash.sort_by { |_,v| v }
  #=> [["a", 100], ["c", 100], ["d", 120], ["b", 200]]

Next, use Enumerable#each_cons to construct an array of all adjacent pairs of elements of b:

c = b.each_cons(2)
  #=> #<Enumerator:
  # [["a", 100], ["c", 100], ["d", 120], ["b", 200]]:each_cons(2)>

To view the contents of this enumerator:

c.to_a
  #=> [[["a", 100], ["c", 100]],
  #    [["c", 100], ["d", 120]],
  #    [["d", 120], ["b", 200]]]

Now build an array consisting of keys to be deleted (duplicates OK)

d = c.each_with_object([]) {
  |((k1,v1),(k2,v2)),a| a << k1 << k2 if (v1-v2).abs <= 50 }
  #=> ["a", "c", "c", "d"]

To compute d, consider the first value passed to the block by the enumerator c:

k1 => "a"
v1 => 100
k2 => "c"
v2 => 100

Since

(100 - 100).abs <= 50

keys k1 and k2 are added to the array of keys to be deleted (block variable a). The next value passed to the block is:

k1 => "c"
v1 => 100
k2 => "d"
v2 => 120

Since

(100 - 120).abs <= 50

the keys "c" and "d" are also added to a. The third value does not add any keys to a since

(120 - 200).abs > 50

Now construct an array of keys to keep by using set difference:

e = hash.keys
  #=> ["a", "b", "c", "d"]

keys_to_keep = e - d
  #=> ["b"]

Pull out the values for the keys to keep, using Hash#values_at:

f = hash.values_at(*keys_to_keep)
  #=> [200]

Construct an array of key/value pairs for keys to keep:

g = keys_to_keep.zip(f)
  #=> [["b", 200]]

Convert to a hash.

g.to_h # Ruby v.2.0+
  #=> {"b"=>200}

or

Hash[g]
  #=> {"b"=>200}