Harrison Wells says that there are "53 Karas" in the multiverse, assuming that every person has a doppelganger in every Earth. This has previously been shown to not be true as Harrison's daughter, Jessie, does not have a doppelganger on Earth-1 and when Barry first arrived at Earth-38 he looked up himself and several other characters from The Flash and Arrow and found that they did not exist on Earth-38.
Kara finds out that Oliver has kryptonite arrows in his inventory. The logistics of this are hard to explain as the substance does not exist on Earth 1, and Team Flash is in possession of the only means of travel to Kara's universe. This means Oliver would need their help in obtaining kryptonite. Moreover, all the kryptonite on Kara's Earth was given to Superman to secure, making it even more unlikely that Oliver would have been able to acquire it. Lastly, Kara is personally offended when she finds out Lena Luthor has kryptonite, but she is far less upset with Oliver for possessing it.