The word in question is the Hebrew word מִשְׁנֵ֤ה mish·neh meaning "a double, copy, second" (http://strongsnumbers.com/hebrew/4932.htm) This word is a very specific word, which Implies a Double Portion, or Twice as Much.. It is Numerical and Precise.
The New World Translation Translates this word as Double in the Below 2 Scriptures :
Isaiah 61:7 "Instead of shame you will have a double portion, And instead of humiliation they will shout joyfully over their share.Yes, they will possess a double (mish·neh) portion in their land. Everlasting rejoicing will be theirs." (The Watchtower applies this Scripture to the "Double" blessings that the Faithful & Discreet Slave will receive because of their "Humiliating" persecution, when they were Imprisoned in 1919. Interestingly this Double Portion of blessings, starts around 1914 according to the WT. (See http://wol.jw.org/en/wol/lv/r1/lp-e/0/20413)
Genesis 43:12 "Take double (mish·neh) the money with you; and also take back the money that was returned in the mouth of your bags.+ Maybe it was a mistake." (The New World Translation also translates the word properly here)
Now See Jeremiah 16:18 in the ESV Translation.. "But first I will doubly (mish·neh) repay their iniquity and their sin, because they have polluted my land with the carcasses of their detestable idols, and have filled my inheritance with their abominations.”
Compare the NWT Version Jeremiah 16:18 "First, I will repay the full amount (mish·neh) due for their error and their sin,For they have profaned my land with the lifeless figures* of their disgusting idolsAnd have filled my inheritance with their detestable things."
In common language "full amount" can mean the same, as "double" however in a Prophetic sense it Absolutely does not work. Why would the Faithful & Discreet Slave purposely mis-translate a Scripture like this though, which obviously applied to God's punishment upon the Israelites?
A quote from the Watchtower's "Old Light" makes the Motive clear. "To our understanding of the Word, Israel's "double" of time was reached in 1878 A.D. from which time thirty seven years of rising in favor will bring them to the year 1915 A.D., to New Covenant conditions promised them through Jeremiah xxxi., 31, compare Romans, xi, 25-32." - ("Jewish Hopes" Watchtower Bible & Tract Society 1910 http://wtarchive.svhelden.info/archive/en/publications/1910_CR_Jewish_Hopes.pdf)
Notice how the Watchtower viewed 1878 as the completion of Double Punishment from God upon the Israelites. (Interestingly 1878 was the Year of "Petah Tikva" http://en.wikipedia.org/wiki/Petah_Tikva or "Opening of Hope") And 37 Years later was according to The 1910 Watchtower Organization, the Restoration of The New Covenant towards Natural Israel! One can see why this may cause some confusion now, seeing that the Current Governing Body, claim that they are the "Spiritual" seed of Abraham "Spiritual Israel"
There is Much More to this Revinsionist History and Purposeful Misinterpration, but It's Clear what their Motive Is.
To further verify that this Translation was Intentional see Isaiah 40:2 "“Speak to the heart of Jerusalem, And proclaim to her that her compulsory service has been fulfilled, That her guilt has been paid off. From the hand of Jehovah she has received a full amount ( kif·la·yim) for all her sins.” - NWT
"Speak tenderly to Jerusalem, and cry to her that her warfare is ended, that her iniquity is pardoned, that she has received from the LORD’s hand double (kif·la·yim) for all her sins." - ESV
The word here kif·la·yim has the Definition of " the double"
Here is what the Watchtower really wants to hide " “Behold, the days are coming, declares the LORD, when I will make a new covenant with the house of Israel and the house of Judah," Jeremiah 31:31