On the surface, red Saber (from Fate/Extra) and blue Saber (from Fate/Stay Night) look almost identical.
Was it ever explained why they look so similar?
What noticeable differences do the two have, aside from their abilities and attire?
@Templar627 makes an interesting distinction in his answer. Personality wise, the Fate/Extra Red Saber is more of a hedonist that does thing overindulgently with much gusto (expressed by her somewhat lewd attire). In the game dialogues and scenes, she is (implied to be) very openly bisexual saying the only things she cares for is what is beautiful ("Beautiful young boys are good. Beautiful young girls are better. My singular preference is beauty!").
Red Saber can be loud and opinionated -- often butting into conversations, throwing around advice, and insulting your enemies as she sees fit. Unlike her blue counterpart, she is vain, boisterous, and arrogant and speaks with all the regal proclamation of an arrogant ruler that is partially divine, not unlike Gilgamesh.
If you consider Blue Saber to be a tsundere, Red Saber would be the complete opposite. She bold (her attire shows off her cleavage and back/behind and legs/leotard/panties) where the other is reserved, proud where the other is humble.
Red Saber seems to be a bit shorter at 150cm tall, compared to the 154cm stature of her Blue counterpart. However Red Saber is bustier (B83/W56/H82) than her Blue counterpart (B73/W53/H76).
In Fate/Extra Material, Red Saber's backstory makes mention that her care for diplomacy caused Britannia's relations in Rome to improve during later years, essentially making her the person who built the opportunity for the legend of King Arthur to be born. Perhaps why she Blue Saber looks so similar to Red Saber is because her legend/archetype was based off Red Saber.
On the wiki-page for Red Saber, there is a little excerpt from a Q&A:
Q: Why are Red Saber and Blue Saber so similar? Are they related?
A: They are very different!
They are different like Takashi Takeuchi and Arco Wada. Or like buckwheat noodles and udon. The model of King Arthur were said to be a fusion of two individuals, and one of them is the Roman general Artorius. I guess there are some relationships.
If I remember correctly, Red Saber as is much more hot blooded and much less proper, as compared to Normal Saber who is much more sedate and proper. Also, Red Saber is much more haughty and acts less humble than Blue Saber. I don't think that it's ever really explained why they look so similar, but the wiki says they don't have any blood relations.
Also, they are two different people, Red Saber is Nero while Blue Saber is Arturia.
If you look at the wiki you can see that they are both extremely different despite their superficial similarities.
On the wiki page that templar627 posted, under the development subheading it says:
She was made to look like Saber because they wanted to trick people into thinking they were the same character and surprise them upon actually playing the game.