There's a well known episode of "Star Trek" that depicts a conflict on a planet that is being visited by the crew of the starship Enterprise. The people on the planet are embroiled in war. One group of people has bright, white skin on the left side of their bodies and black skin on the right side. The second group has white skin on the right side and black on the left. Because of this, they are gripped in bitter war. And as we know, "Star Trek" aired in the 1960s, during time of great racial strife. And this is what we call hitting readers (or viewers) over the head with a metaphor.
Also known as being heavy-handed, hitting your readers over the head with your point is a sure way to get that point across. But is it also a sure way to annoy your readers?
Beating a Dead Horse
There are lots of metaphors for overstating a point, even when it's a plot point in a story. That's because it's a thing that many people don't like.
However, re-stating a plot point is a truly time-honored tradition in storytelling. The oldest stories ever told were not written down, they were passed along orally, spoken aloud into groups. In order to make sure specific information is stuck in listeners’ minds, storytellers would repeat the same information over and over again. This style of storytelling can still be seen as a lingering tradition in many of the oldest written stories. Homer's "Odyssey," for example, is repetitive on several points.
There are tens of thousands of words in the average book and certain information will stick with some readers and not with others. If you want to make sure that they retain specific points of information, repetition is the best way to make that happen. However, you can be repetitive without being boring.
You can present the same information over and over again but in different ways, using different words. If it is important to know that a character is a coward, for example, show the readers a cowardly act committed by this character. You can also show the reader the character's thoughts about the event afterwards and in this fashion, show the cowardly action again in a slightly different way. There are many ways to give the readers the same information. You can also do it through dialogue by having a character relate a story to another character and have the character repeat the tale, using somewhat different words, to yet another character.
You can also use an extremely transparent metaphor, such as the one used in the "Star Trek" episode mentioned above, to make sure a point sticks out. When the point is thinly-hidden like this, you won't need to use repetition to get the point across. Readers may roll their eyes, but you as the writer don't have to worry that they won't "get it." They will.
Sometimes, hitting readers over the head may be a necessary evil. But if you do it well and craft a story that readers love, they won't mind.