When you square an expression with dummy indices, you must rename them first. People often do this by hand, e.g. as in
ex1 = (FV[p, \[Mu]] + FV[q, \[Mu]]) FV[r, \[Mu]] FV[r, \[Nu]]\overline{r}^{\mu } \overline{r}^{\nu } \left(\overline{p}^{\mu }+\overline{q}^{\mu }\right)
ex1 (ex1 /. \[Mu] -> \[Rho])
Contract[%]\overline{r}^{\mu } \left(\overline{r}^{\nu }\right)^2 \overline{r}^{\rho } \left(\overline{p}^{\mu }+\overline{q}^{\mu }\right) \left(\overline{p}^{\rho }+\overline{q}^{\rho }\right)
\overline{r}^2 \left(\overline{p}\cdot \overline{r}+\overline{q}\cdot \overline{r}\right)^2
However, since FeynCalc 9 there is a function for that
FCRenameDummyIndices[ex1]\overline{r}^{\nu } \overline{r}^{\text{\$AL}(\text{\$19})} \left(\overline{p}^{\text{\$AL}(\text{\$19})}+\overline{q}^{\text{\$AL}(\text{\$19})}\right)
ex1 FCRenameDummyIndices[ex1]
Contract[%]\overline{r}^{\mu } \overline{r}^{\nu } \overline{r}^{\nu } \left(\overline{p}^{\mu }+\overline{q}^{\mu }\right) \overline{r}^{\text{\$AL}(\text{\$20})} \left(\overline{p}^{\text{\$AL}(\text{\$20})}+\overline{q}^{\text{\$AL}(\text{\$20})}\right)
\overline{r}^2 \left(\overline{p}\cdot \overline{r}+\overline{q}\cdot \overline{r}\right)^2
Notice that FCRenameDummyIndices does not canonicalize the indices
FV[p, \[Nu]] FV[q, \[Nu]] - FV[p, \[Mu]] FV[q, \[Mu]]
FCRenameDummyIndices[%]\overline{p}^{\nu } \overline{q}^{\nu }-\overline{p}^{\mu } \overline{q}^{\mu }
\overline{p}^{\text{\$AL}(\text{\$22})} \overline{q}^{\text{\$AL}(\text{\$22})}-\overline{p}^{\text{\$AL}(\text{\$21})} \overline{q}^{\text{\$AL}(\text{\$21})}
But since FeynCalc 9.1 there is a function for that too
FV[p, \[Nu]] FV[q, \[Nu]] - FV[p, \[Mu]] FV[q, \[Mu]]
FCCanonicalizeDummyIndices[%]\overline{p}^{\nu } \overline{q}^{\nu }-\overline{p}^{\mu } \overline{q}^{\mu }
0
Finally, often we also need to uncontract already contracted indices. This is done by Uncontract. By default, it handles only contractions with Dirac matrices and Levi-Civita tensors
LC[][p, q, r, s]
Uncontract[%, p]
Uncontract[%%, p, q]\bar{\epsilon }^{\overline{p}\overline{q}\overline{r}\overline{s}}
\overline{p}^{\text{\$AL}(\text{\$31})} \bar{\epsilon }^{\text{\$AL}(\text{\$31})\overline{q}\overline{r}\overline{s}}
\overline{p}^{\text{\$AL}(\text{\$33})} \overline{q}^{\text{\$AL}(\text{\$32})} \left(-\bar{\epsilon }^{\text{\$AL}(\text{\$32})\text{\$AL}(\text{\$33})\overline{r}\overline{s}}\right)
SP[p, q]
Uncontract[%, p]\overline{p}\cdot \overline{q}
\overline{p}\cdot \overline{q}
To uncontract scalar products as well, use the option Pair->All
Uncontract[%, p, Pair -> All]\overline{p}^{\text{\$AL}(\text{\$34})} \overline{q}^{\text{\$AL}(\text{\$34})}