Identical letters cannot be next to each other vertically or horizontally, but may be adjacent diagonally.

                 +---+---+---+---+---+
              A  | 2 | 2 | 0 | 1 | 1 |
                 +---+---+---+---+---+
              B  | 1 | 1 | 2 | 2 | 0 |
                 +---+---+---+---+---+
              C  | 2 | 1 | 1 | 0 | 1 |
                 +---+---+---+---+---+
  A   B   C   D  | 0 | 1 | 2 | 2 | 3 |
+---+---+---+---++===+===+===+===+===+
| 0 | 2 | 1 | 2 ||   |   |   |   |   |
+---+---+---+---++---+---+---+---+---+
| 1 | 1 | 2 | 1 ||   |   |   |   |   |
+---+---+---+---++---+---+---+---+---+
| 2 | 0 | 2 | 1 ||   |   |   |   |   |
+---+---+---+---++---+---+---+---+---+
| 2 | 1 | 0 | 2 ||   |   |   |   |   |
+---+---+---+---++---+---+---+---+---+
| 1 | 2 | 0 | 2 ||   |   |   |   |   |
+---+---+---+---++---+---+---+---+---+