Synastry is a part of relationship astrology, and you take the natal charts of two people and compare them to one another. This helps you to see how compatible they are with one another, how their charts impact one another, and how they each make the other feel. When you see repetition between the charts, where both people have the same of something in their chart (like the same aspect between planets, same aspect pattern/configuration with the same planets/elements/houses involved, same placement of planets in houses, or same planets at certain degrees), this can be either a good thing or a bad thing. The good part is that it lends itself to strengthening understanding between the two people because they both have a natural understanding of the energy and impact of whatever it is they have that’s the same. The bad part is that it means you can both play up the same bad manifestations of it, and this can be a major weakness in the relationship or partnership.
This can be most obvious when it comes to planets at critical or anaretic degrees. These planets often come with major challenges for the native to overcome, and can be a weak spot in the chart until they do. To encounter someone else with the same planet also at a critical or anaretic degree usually shows greater understanding about this weak spot, but can also show that in the relationship or partnership, this is a problem. For example, in a couple that has both of their Saturns at critical or anaretic degrees, you may be looking at a couple that is so irresponsible that they ruin their lives together. In a couple where both have their Moons at a critical or anaretic degree, they may have such emotional difficulties that they’re constantly fueling one another to a deeply emotional, disturbing place.
Having the same aspect occur as another person between planets can be a point of understanding, but depending on the aspect and planets involved, it can contribute to conflict. For example, in a couple where both have Mars square Pluto, the arguments may be far more intense, even violent. In a couple where both have Sun opposite Uranus, there can be such erratic energy between them that it’s difficult to control.
Having planets in the same houses can be more helpful when it comes to most of the planets; the locations of Saturn, Uranus, Neptune, and Pluto tend to be more challenging, so having them in the same houses can show extra limitations and responsibilities (Saturn), extra instability (Uranus), extra delusion (Neptune), and extra power struggles (Pluto).
Having the same aspect pattern/configuration in your charts can be more helpful if they’re not in the same signs or houses or involve the same planets. This can give enough of a difference that you have a subtle understanding of the pattern itself without both of you having the exact same weak spot. Having the same apex planet or missing sign/house in a t-square, a grand square in the same quality or with the same planets, or the same planet as a singleton can be most problematic and require much more effort and focus. Interestingly, the grand trine, which is traditionally viewed a positive, can be difficult for a couple if they have the same planets or the same element involved because it can make things too easy and be a source of major laziness.