How to remove duplicate commands from irb history?

Question

I have searched several questions/answers/blogs without success. How to remove/delete duplicate commands from irb history?

Ideally I want to have the same behavior I configured for my bash. That is: after I execute a command every other entry in the history with the exactly same command is deleted.

But it would already be good to eliminate duplicates when I close irb.

My current .irbrc:

require 'irb/ext/save-history'
IRB.conf[:SAVE_HISTORY] = 1000
IRB.conf[:HISTORY_FILE] = "#{ENV['HOME']}/.irb_history"
IRB.conf[:AUTO_INDENT] = true

Note: Ruby 2.4.1 (or newer!)

Answer 1

This will eliminate duplicates after closing IRB console. But it works only for IRBs using Readline (mac users warned).

# ~/.irbrc
require 'irb/ext/save-history'
IRB.conf[:SAVE_HISTORY] = 1000
IRB.conf[:HISTORY_FILE] = "#{ENV['HOME']}/.irb_history"

deduplicate_history = Proc.new do
    history = Readline::HISTORY.to_a
    Readline::HISTORY.clear
    history.reverse!.uniq!
    history.reverse!.each{|entry| Readline::HISTORY << entry}
end

IRB.conf[:AT_EXIT].unshift(deduplicate_history)

And this monkey patch will eliminate duplicates on the fly if your IRB is using Readline:

require 'irb/ext/save-history'
IRB.conf[:SAVE_HISTORY] = 1000
IRB.conf[:HISTORY_FILE] = "#{ENV['HOME']}/.irb_history"

class IRB::ReadlineInputMethod
    alias :default_gets :gets
    def gets
        if result = default_gets
            line = result.chomp
            history = HISTORY.to_a
            HISTORY.clear
            history.each{|entry| HISTORY << entry unless entry == line}
            HISTORY << line
        end
        result
    end
end

Any suggestion on how to improve it?