There are lots of aspects which help to make a strong composition. One of them is REPETITION.
This is a really fundamental part of almost every form of design. The repetition of an element in a picture can help to give a quality of UNITY, which will help the whole of a complex design to "hang together". It is perhaps comparable to the repeating phrases in a musical score, which help to create something satisfactory to listen to, rather than a random set of unconnected tones.
Repetition is not, however, the same as "repetitiveness", what we want to avoid is monotony. We need some variety with our unity. Our eyes will enjoy a repeated form, or group of forms a little different from each other, but similar, such as the irregular arches in an old bridge.
Shapes, or groups of shapes, can be echoed - or another way to think of it is "rhymed" - in different parts of a picture with great subtlety, so that initially the connection is not that obvious. In a landscape, for example, a small form can imitate a large and important form - a line of trees could be the same shape as a very large cloud or set of clouds. Repetition can also be used in the re-duplication of colours throughout an image.
Formal devices of this kind are important tools in the painter's arsenal of tools. That said...sometimes, the painter does not consciously decide to do this or that...it happens as the work evolves. However, there is no getting away from the fact that understanding the power of these helpful concepts and ideas will enable us to see and catch hold of possibilities as we work - sometimes as we produce our initial thumbnail sketches, sometimes as our work is in progress.
Let's take a look at a very obvious form of repetition. Here we have Renoir's The Umbrellas, painted c 1872-82. Look at the obvious repetition of the umbrellas, but notice also how the curving forms are repeated elsewhere....the hat of the girl on the right, the curving form of the central figure's face, the top edge of the basket - the hoop held by the little girl , less obvious but still there -the bottom of the jacket on the woman on the left and the folds in her dress - the whole picture is a mass of echoing forms, curves either curving down, or curving up.