1. A method, implemented by one or more processors of a computing system, for efficiently grouping electronic documents that are likely textual near-duplicates, the method comprising: processing, by the one or more processors, a first electronic document and a second electronic document to determine respective sets of character sequence counts, wherein processing the first electronic document and the second electronic document includes, for each electronic document, identifying a plurality of non-contiguous character sequences expressed within text of the electronic document, each non-contiguous character sequence corresponding to a different starting word position within the text and including (i) a first character at a first pre-defined character position within a word at the corresponding starting word position, and (ii) a second character at a second pre-defined character position within a word that is offset from the corresponding starting word position by a pre-defined number of words, and determining a set of character sequence counts, at least in part by determining, for each unique non-contiguous character sequence occurring within the identified plurality of non-contiguous character sequences, a character sequence count specifying how many times the unique non-contiguous character sequence is expressed within the text of the electronic document; generating, by the one or more processors, one or more similarity metrics, wherein generating the one or more similarity metrics includes comparing (i) the set of character sequence counts determined for the first electronic document to (ii) the set of character sequence counts determined for the second electronic document; calculating, by the one or more processors and using the one or more similarity metrics, a similarity score for the first electronic document and the second electronic document; and assigning, by the one or more processors and based on the similarity score, the second electronic document to a same document group as the first electronic document.