What was the “double oppression” that Katherine, Dorothy, and Mary were fighting against?

the third paragraph is your answer,

"but, in a larger sense, we can not dedicate — we can not consecrate — we can not hallow — this ground. the brave men, living and dead, who struggled here, have consecrated it, far above our poor power to add or detract. the world will little note, nor long remember what we say here, but it can never forget what they did here. it is for us the living, rather, to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced. it is rather for us to be here dedicated to the great task remaining before us— that from these honored dead we take increased devotion to that cause for which they gave the last full measure of devotion—that we here highly resolve that these dead shall not have died in vain—that this nation, under god, shall have a new birth of freedom—and that government of the people, by the people, for the people, shall not perish from the earth. "

edwards uses the phrase "o who can express what the state of a soul in such circumstances is! " in order to illustrate that this type of suffering is unimaginable. the word "express" denoted that the state of a suffering soul is required to be described by any person, but it would not happen because no one can imagine how this suffering is felt.  

-show what might happen if the reader disobeys edwards is not correct because this is described trhough the text, not by the phrase used.

-describe what endless suffering looks and feels like is not correct because this description might be the porpuse of the excerpt provided, not the porpuse of using the phrase during the writting.

-prove that few people have survived limitless suffering is not correct because it is implied that the soul of a person is already suffering and doesn't have any way to escape.