Delete duplicate lines

Found by inspecting results of running this small program:

int main(int argc, const char **argv) {
	for (int i = 1; i < argc; i++) {
		FILE *f;
		char line[1024], prev[1024], *r;
		int lineno;

		prev[0] = '\0';
		lineno = 1;
		f = fopen(argv[i], "r");
		if (!f)
			exit(1);
		do {
			r = fgets(line, sizeof(line), f);
			if (!r)
				break;
			if (strcmp(line, prev) == 0)
				printf("%s:%d: error: dup %s", argv[i], lineno, line);
			lineno++;
			strcpy(prev, line);
		} while (!feof(f));
		fclose(f);
	}
}